import collections

def read_data():
    N, M, K = map(int, input().split())
    Es = [collections.defaultdict(set) for i in range(N)]
    roads_v = collections.defaultdict(set)
    for m in range(M):
        a, b, c = map(int, input().split())
        a -= 1
        b -= 1
        Es[a][c].add(b)
        Es[b][c].add(a)
        roads_v[c].add(a)
        roads_v[c].add(b)
    ds = list(map(int, input().split()))
    return N, M, K, Es, roads_v, ds

def solve(N, M, K, Es, roads_v, ds):
    candidates = roads_v[ds[0]]
    for d in ds[1:]:
        new_candidates = set()
        for src in candidates:
            new_candidates |= Es[src][d]
        candidates = new_candidates
    candidates = list(candidates)
    candidates.sort()
    candidates = [c + 1 for c in candidates]
    print(len(candidates))
    print(*candidates)

N, M, K, Es, roads_v, ds = read_data()
solve(N, M, K, Es, roads_v, ds)