using A;
using System.Buffers;
using System.Collections;
using System.Diagnostics;
using System.Numerics;
using System.Runtime.CompilerServices;
using static A.InputUtility;
class Program
{
    static void Main()
    {
        using Output output = new(false);

        InputNewLine();
        var t = NextInt32;

        for (int _ = 0; _ < t; _++)
        {
            InputNewLine();
            var (n, l) = (NextInt32, NextInt32);
            var a = new (int x, int y)[n];
            for (int i = 0; i < a.Length; i++)
            {
                InputNewLine();
                a[i] = (NextInt32, NextInt32);
            }

            (int x, int y)[] res = Solve(n, l, a);

            Console.WriteLine(res.Length);
            foreach (var (x, y) in res)
            {
                Console.WriteLine($"{x} {y}");
            }
        }
    }

    private static (int x, int y)[] Solve(int n, int l, (int x, int y)[] a)
    {
        var max = a.Max(x => x.y);
        var b = (int)Math.Sqrt(max);

        var c = a.GroupBy(x => x.y / b).OrderBy(x => x.Key).Select(x => x.OrderBy(x => x.x).ToArray()).ToArray();

        List<(int x, int y)> list = new();
        for (int i = 0; i < c.Length; i++)
        {
            if ((i & 1) == 0)
            {
                list.AddRange(c[i]);
            }
            else
            {
                list.AddRange(c[i].Reverse());
            }
        }

        var res = list.ToArray();
        return res;
    }
}

namespace A
{
    public static class InputUtility
    {
        private static string[]? s_inputs;
        private static string? s_raw;
        private static int s_index = 0;

        private static void Init() => s_index = 0;
        public static int NextInt32 => int.Parse(s_inputs![s_index++]!);
        public static uint NextUInt32 => uint.Parse(s_inputs![s_index++]!);
        public static long NextInt64 => long.Parse(s_inputs![s_index++]!);
        public static ulong NextUInt64 => ulong.Parse(s_inputs![s_index++]!);
        public static string NextString => s_inputs![s_index++];
        public static char NextChar => s_inputs![s_index++][0];
        public static decimal NextDecimal => decimal.Parse(s_inputs![s_index++]!);
        public static BigInteger NextBigInteger => BigInteger.Parse(s_inputs![s_index++]!);
        public static int[] GetInt32Array() => s_inputs!.Select(int.Parse).ToArray();
        public static long[] GetInt64Array() => s_inputs!.Select(long.Parse).ToArray();
        public static string GetRawString() => s_raw!;
#if DEBUG
        private static TextReader? s_textReader;
        public static void SetSource(string path) => s_textReader = new StringReader(File.ReadAllText(path));
#endif
        public static bool InputNewLine()
        {
#if DEBUG
            if (s_textReader is TextReader sr)
            {
                Init();
                s_raw = sr.ReadLine()!;
                s_inputs = s_raw.Split(' ', StringSplitOptions.RemoveEmptyEntries);
                return true;
            }
#endif
            Init();
            s_raw = Console.ReadLine()!;
            s_inputs = s_raw.Split(' ', StringSplitOptions.RemoveEmptyEntries);
            return true;
        }
    }

    public static class CombinationFunction
    {
        public static void PartedRotate<T>(T[] a, int first1, int last1, int first2, int last2)
        {
            if (first1 == last1 || first2 == last2) return;
            int next = first2;
            while (first1 != next)
            {
                Swap(a, first1++, next++);
                if (first1 == last1) first1 = first2;
                if (next == last2)
                {
                    next = first2;
                }
                else if (first1 == first2)
                {
                    first2 = next;
                }
            }
        }

        public static bool NextCombinationImp<T>(T[] a, int first1, int last1, int first2, int last2) where T : IComparable<T>
        {
            if (first1 == last1 || first2 == last2) return false;
            int target = last1 - 1;
            int lastElem = last2 - 1;

            while (target != first1 && !(a[target].CompareTo(a[lastElem]) < 0)) target--;
            if (target == first1 && !(a[target].CompareTo(a[lastElem]) < 0))
            {
                PartedRotate(a, first1, last1, first2, last2);
                return false;
            }

            int next = first2;
            while (!(a[target].CompareTo(a[next]) < 0)) next++;
            Swap(a, target++, next++);
            PartedRotate(a, target, last1, next, last2);
            return true;
        }

        public static bool NextCombination<T>(T[] a, int first, int mid, int last) where T : IComparable<T>
            => NextCombinationImp(a, first, mid, mid, last);

        public static bool PrevCombination<T>(T[] a, int first, int mid, int last) where T : IComparable<T>
            => NextCombinationImp(a, mid, last, first, mid);

        public static void Swap<T>(T[] a, int i, int j) => (a[i], a[j]) = (a[j], a[i]);
    }

    public static class PermutationFunction
    {
        public static bool NextPermutation<T>(T[] a) where T : IComparable<T>
        {
            int n = a.Length;

            int i = n - 2;
            while (i >= 0 && a[i].CompareTo(a[i + 1]) >= 0) { i--; }

            if (i < 0) { return false; }

            int j = n - 1;
            while (a[j].CompareTo(a[i]) <= 0) { j--; }
            (a[i], a[j]) = (a[j], a[i]);

            Array.Reverse(a, i + 1, n - i - 1);

            return true;
        }
    }

    public readonly struct Output : IDisposable
    {
        private readonly StreamWriter _sw;
#if DEBUG
        public Output(string path)
        {
            var fs = new FileStream(path, FileMode.Create, FileAccess.Write);
            _sw = new StreamWriter(fs);
            Console.SetOut(_sw);
        }
#endif
        public Output(bool autoFlush)
        {
            _sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = autoFlush };
            Console.SetOut(_sw);
        }

        public void Dispose()
        {
            _sw.Dispose();
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        internal void Flush()
        {
            _sw.Flush();
        }
    }

    public static class ArrayExtensions
    {
        public static void Swap<T>(this T[] array, int i, int j) => (array[i], array[j]) = (array[j], array[i]);

        public static int LowerBound<T>(this T[] a, T target) where T : IComparable<T>
        {
            int ok = a.Length;
            int ng = -1;

            while (Math.Abs(ok - ng) > 1 && (ok + ng) / 2 is int mid)
                (ok, ng) = a[mid].CompareTo(target) >= 0 ? (mid, ng) : (ok, mid);

            return ok;
        }

        public static int UpperBound<T>(this T[] a, T target) where T : IComparable<T>
        {
            int ok = a.Length;
            int ng = -1;

            while (Math.Abs(ok - ng) > 1 && (ok + ng) / 2 is int mid)
                (ok, ng) = a[mid].CompareTo(target) > 0 ? (mid, ng) : (ok, mid);

            return ok;
        }

        public struct IndexedEnumerable<T> : IEnumerable<(T item, int index)>
        {
            private readonly T[] _a;
            private readonly int _startIndex;

            public IndexedEnumerable(T[] a, int startIndex = 0)
            {
                _a = a;
                _startIndex = startIndex;
            }

            public readonly IndexedEnumerator<T> GetEnumerator() => new IndexedEnumerator<T>(_a, _startIndex);
            IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
            IEnumerator<(T item, int index)> IEnumerable<(T item, int index)>.GetEnumerator() => GetEnumerator();
        }

        public struct IndexedEnumerator<T> : IEnumerator<(T item, int index)>
        {
            public readonly (T item, int index) Current => (_a[_index], _index + _startIndex);

            private int _index;
            private int _startIndex;
            private T[] _a;

            public IndexedEnumerator(T[] a, int startIndex)
            {
                _index = -1;
                _a = a;
                _startIndex = startIndex;
            }

            public bool MoveNext() => ++_index < _a.Length;
            readonly object IEnumerator.Current => Current;
            public readonly void Dispose() { }
            public void Reset() => _index = -1;
        }

        /// <returns>(T value, int index)</returns>
        public static IndexedEnumerable<T> Enumerate<T>(this T[] arr, int startIndex = 0) => new IndexedEnumerable<T>(arr, startIndex);
    }

    public static class IEnumerableExtensions
    {
        public static IEnumerable<TSource> Log<TSource>(this IEnumerable<TSource> source)
        {
            Console.WriteLine(string.Join(' ', source));
            return source;
        }

        public static ScanEnumerable<TSource, TAccumulate> Scan<TSource, TAccumulate>(
            this IEnumerable<TSource> source,
            TAccumulate seed,
        Func<TAccumulate, TSource, TAccumulate> accumulator) where TSource : struct where TAccumulate : struct
        {
            return new ScanEnumerable<TSource, TAccumulate>(source, accumulator, seed);
        }

        public static IEnumerable<TAccumulate> ScanExSeed<TSource, TAccumulate>(
            this IEnumerable<TSource> source,
            TAccumulate seed,
            Func<TAccumulate, TSource, TAccumulate> accumulator)
        {
            var accumulation = new List<TAccumulate>();

            var current = seed;
            foreach (var item in source)
            {
                current = accumulator(current, item);
                accumulation.Add(current);
            }

            return accumulation;
        }

        public readonly struct ScanEnumerable<TSource, TAccumulate> : IEnumerable<TAccumulate> where TSource : struct where TAccumulate : struct
        {
            private readonly IEnumerable<TSource> _source;
            private readonly Func<TAccumulate, TSource, TAccumulate> _accumulator;
            private readonly TAccumulate _seed;

            public ScanEnumerable(IEnumerable<TSource> source, Func<TAccumulate, TSource, TAccumulate> accumulator, TAccumulate seed)
            {
                _source = source;
                _accumulator = accumulator;
                _seed = seed;
            }

            public readonly ScanEnumerator<TSource, TAccumulate> GetEnumerator() => new(_source, _accumulator, _seed);
            readonly IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
            readonly IEnumerator<TAccumulate> IEnumerable<TAccumulate>.GetEnumerator() => GetEnumerator();
        }

        public struct ScanEnumerator<TSource, TAccumulate> : IEnumerator<TAccumulate> where TSource : struct where TAccumulate : struct
        {
            private readonly Func<TAccumulate, TSource, TAccumulate> _accumulator;

            private readonly IEnumerator<TSource> _enumerator;
            private TAccumulate _current;

            private bool _secondOrLaterElement = false;

            public ScanEnumerator(IEnumerable<TSource> source, Func<TAccumulate, TSource, TAccumulate> accumulator, TAccumulate seed)
            {
                _enumerator = source.GetEnumerator();
                _accumulator = accumulator;

                _current = seed;
            }

            public readonly TAccumulate Current => _current;
            readonly object IEnumerator.Current => Current;

            public readonly void Dispose() { }

            public bool MoveNext()
            {
                if (_secondOrLaterElement)
                {
                    if (_enumerator.MoveNext())
                    {
                        _current = _accumulator(_current, _enumerator.Current);
                        return true;
                    }
                    return false;
                }
                else
                {
                    _secondOrLaterElement = true;
                    return true;
                }
            }

            public void Reset()
            {
                throw new NotSupportedException();
            }
        }

        public static IEnumerable<TSource> Scan<TSource>(
            this IEnumerable<TSource> source,
            Func<TSource, TSource, TSource> accumulator)
        {
            if (source is null)
                throw new ArgumentNullException(paramName: nameof(source));

            if (accumulator is null)
                throw new ArgumentNullException(paramName: nameof(accumulator));

            var accumulation = new List<TSource>();

            if (source.Any() is false)
            {
                return accumulation;
            }

            var current = source.First();
            accumulation.Add(current);

            foreach (var item in source.Skip(1))
            {
                current = accumulator(current, item);
                accumulation.Add(current);
            }

            return accumulation;
        }

        public static CombinationEnumerable<T> Combination<T>(this T[] a, int k) where T : IComparable<T>
            => new(a, k);

        public readonly struct CombinationEnumerable<T> where T : IComparable<T>
        {
            private readonly T[] _a;
            private readonly int _k;

            public CombinationEnumerable(T[] a, int k)
            {
                _a = a;
                _k = k;
            }

            public readonly CombinationEnumerator<T> GetEnumerator() => new(_a, _k);
        }

        public struct CombinationEnumerator<T> : IEnumerator<ReadOnlyMemory<T>> where T : IComparable<T>
        {
            private readonly int _k;
            private readonly T[] _a;
            private readonly int _n;
            private bool _secondOrLaterElement = false;

            public CombinationEnumerator(T[] a, int k)
            {
                _a = a;
                _n = a.Length;
                _k = k;
            }

            public readonly ReadOnlyMemory<T> Current => _a.AsMemory()[.._k];
            readonly object IEnumerator.Current => Current;

            public readonly void Dispose() { }

            public bool MoveNext()
            {
                if (_secondOrLaterElement)
                {
                    return CombinationFunction.NextCombination(_a, 0, _k, _n);
                }
                else
                {
                    _secondOrLaterElement = true;
                    return true;
                }
            }

            public void Reset()
            {
                throw new NotSupportedException();
            }
        }

        public static PermutationEnumerable<T> Permutation<T>(this T[] a) where T : IComparable<T> => new(a);

        public readonly struct PermutationEnumerable<T> where T : IComparable<T>
        {
            private readonly T[] _a;
            public PermutationEnumerable(T[] a) => _a = a;
            public readonly PermutationEnumerator<T> GetEnumerator() => new(_a);
        }

        public struct PermutationEnumerator<T> : IEnumerator<ReadOnlyMemory<T>> where T : IComparable<T>
        {
            private readonly T[] _a;
            private readonly int _n;
            private bool _secondOrLaterElement = false;

            public PermutationEnumerator(T[] a)
            {
                _a = a;
                _n = a.Length;
            }
            public readonly ReadOnlyMemory<T> Current => _a.AsMemory()[..];
            readonly object IEnumerator.Current => Current;

            public readonly void Dispose() { }

            public bool MoveNext()
            {
                if (_secondOrLaterElement)
                {
                    return PermutationFunction.NextPermutation(_a);
                }
                else
                {
                    _secondOrLaterElement = true;
                    return true;
                }
            }

            public void Reset() => throw new NotSupportedException();
        }
    }

    public class MyMath
    {
        public static long Pow(long x, int y) => Enumerable.Repeat(x, y).Aggregate(1L, (acc, x) => acc * x);
        public static long Pow10(int y) => Pow(10, y);
        public static long Pow2(int y) => Pow(2, y);
    }

    public class MyMathBigInteger
    {
        public static BigInteger Pow(long x, int y) => Enumerable.Repeat(x, y).Aggregate(new BigInteger(1), (acc, x) => acc * x);
        public static BigInteger Pow10(int y) => Pow(10, y);
        public static BigInteger Pow2(int y) => Pow(2, y);
    }
}
#region Expanded by https://github.com/kzrnm/SourceExpander
namespace SourceExpander{public class Expander{[Conditional("EXP")]public static void Expand(string inputFilePath=null,string outputFilePath=null,bool ignoreAnyError=true){}public static string ExpandString(string inputFilePath=null,bool ignoreAnyError=true){return "";}}}
#endregion Expanded by https://github.com/kzrnm/SourceExpander