N, M, K = [int(i) for i in input().strip().split(" ")]

fee = {}
for i in range(M):
    a, b, c = [int(i) for i in input().strip().split(" ")]
    if fee.get(c):
        fee[c].append((a,b))
    else:
        fee[c] = [(a,b)]

D = [int(i) for i in input().strip().split(" ")]

cur = set([])
towns = fee[D[0]]
for town in towns:
    cur.add(town[0])
    cur.add(town[1])

next = set([])
for d in D[1:]:
    towns = fee[d]
    for town in towns:
        if town[0] in cur:
            next.add(town[1])
        if town[1] in cur:
            next.add(town[0])
    cur = next
    next = set([])

cur = list(cur)
print(len(cur))
print(" ".join([str(i) for i in cur]))