from collections import defaultdict

def main():
    N, M, K = map(int, input().split())

    # 料金ごとに道路をまとめる
    edges_by_cost = defaultdict(list)
    for _ in range(M):
        a, b, c = map(int, input().split())
        edges_by_cost[c].append((a, b))

    d = list(map(int, input().split()))

    # 1回目の通行料金
    possible = set()
    for a, b in edges_by_cost[d[0]]:
        possible.add(a)
        possible.add(b)

    # 2回目以降
    for i in range(1, K):
        next_possible = set()
        for a, b in edges_by_cost[d[i]]:
            if a in possible:
                next_possible.add(b)
            if b in possible:
                next_possible.add(a)
        possible = next_possible

    # 出力
    result = sorted(possible)
    print(len(result))
    print(" ".join(map(str, result)))

if __name__ == "__main__":
    main()