import sequtils,algorithm,strutils,intsets,tables,sugar
template times*(n:int,body) = (for _ in 0..<n: body)

proc getchar_unlocked():char {. importc:"getchar_unlocked",header: "<stdio.h>" .}
proc scan(): int =
  while true:
    let k = getchar_unlocked()
    if k < '0': break
    result = 10 * result + k.ord - '0'.ord


let n = scan()
let m = scan()
let k = scan()
let ABC = newSeqWith(m,(a:scan(),b:scan(),c:scan())).sortedByIt(it.c)
var oks = initIntSet()
for i in 1..n: oks.incl i
k.times:
  var nexts = initIntSet()
  let d = scan()
  let iMin = ABC.binarySearch(d,(x,y)=>x.c-y)
  for i in iMin..<m:
    let abc = ABC[i]
    if abc.c != d: break
    if abc.a in oks: nexts.incl abc.b
    if abc.b in oks: nexts.incl abc.a
  oks = nexts
echo oks.card
echo toSeq(oks.items).sorted(cmp).mapIt($it).join(" ")