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

const INF = 10000
proc encode(x,y:int):int = x * INF + y
proc decodeX(i:int):int = i mod INF
proc decodeY(i:int):int = i div INF

let n = scan()
let m = scan()
let k = scan()
var P = newTable[int,IntSet]()
m.times:
  let a = scan()
  let b = scan()
  let c = scan()
  if c notin P : P[c] = initIntSet()
  P[c].incl encode(a,b)
let D = newSeqWith(k,scan())
var nows = initIntSet()
for p in P[D[0]]:
  nows.incl p.decodeX()
  nows.incl p.decodeY()
for i in 1..<k:
  var nexts = initIntSet()
  for p in P[D[i]]:
    let x = p.decodeX()
    let y = p.decodeY()
    if x in nows: nexts.incl y
    if y in nows: nexts.incl x
  nows = nexts
echo nows.card
echo toSeq(nows.items).sorted(cmp).mapIt($it).join(" ")