import sequtils,algorithm,strutils,intsets,tables
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()
var A = newSeq[int](m)
var B = newSeq[int](m)
var C = newSeq[int](m)
for i in 0..<m:
  A[i] = scan()
  B[i] = scan()
  C[i] = scan()
var oks = initIntSet()
for i in 1..n: oks.incl i
k.times:
  var nexts = initIntSet()
  let d = scan()
  for i in 0..<m:
    if C[i] != d: continue
    if A[i] in oks: nexts.incl B[i]
    if B[i] in oks: nexts.incl A[i]
  oks = nexts
echo oks.card
echo toSeq(oks.items).sorted(cmp).mapIt($it).join(" ")