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 P = newTable[int,seq[seq[bool]]]()
m.times:
  let a = scan()
  let b = scan()
  let c = scan()
  if c notin P : P[c] = newSeqWith(n+1,newSeq[bool](n+1))
  P[c][a][b] = true
  P[c][b][a] = true
let D = newSeqWith(k,scan())
var answers = initIntSet()
let p0 = P[D[0]]
for a in 1..n:
  for b in (a+1)..n:
    if not p0[a][b] : continue
    answers.incl a
    answers.incl b
for i in 1..<k:
  let pn = P[D[i]]
  var nexts = initIntSet()
  for a in answers:
    for b in 1..n:
      if not pn[a][b] : continue
      nexts.incl b
  answers = nexts
echo answers.card
echo toSeq(answers.items).sorted(cmp).mapIt($it).join(" ")