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[tuple[a,b:int]]]()
m.times:
  let a = scan()
  let b = scan()
  let c = scan()
  if c notin P : P[c] = @[]
  P[c] &= (a,b)
let D = newSeqWith(k,scan())
var answers = initIntSet()
for p in P[D[0]]:
  answers.incl p.a
  answers.incl p.b
for i in 1..<k:
  var nexts = initIntSet()
  for p in P[D[i]]:
    if p.a in answers:
      nexts.incl p.b
    if p.b in answers:
      nexts.incl p.a
  answers = nexts
echo answers.card
echo toSeq(answers.items).sorted(cmp).mapIt($it).join(" ")