#!/usr/bin/python
from collections import defaultdict

n, m, k = map(int, raw_input().split())
G = defaultdict(list)
for _ in xrange(m):
    a, b, c = map(int, raw_input().split())
    G[c].append((a-1, b-1))
    G[c].append((b-1, a-1))
d = map(int, raw_input().split())
arr = set(xrange(n))
for x in d:
    arr = {to for fra, to in G[x] if fra in arr}
res = list(map(lambda e: e+1, arr))
print len(res)
print ' '.join(map(str, res))