#include<bits/stdc++.h>

using namespace std;
typedef unsigned int uint;
typedef long long int ll;
typedef unsigned long long int ull;

#define debugv(v) printf("L%d %s => ",__LINE__,#v);for(auto e:v){cout<<e<<" ";}cout<<endl;
#define debugm(m) printf("L%d %s is..\n",__LINE__,#m);for(auto v:m){for(auto e:v){cout<<e<<" ";}cout<<endl;}
#define debuga(m,w) printf("L%d %s is => ",__LINE__,#m);for(int x=0;x<(w);x++){cout<<(m)[x]<<" ";}cout<<endl;
#define debugaa(m,w,h) printf("L%d %s is..\n",__LINE__,#m);for(int y=0;y<(h);y++){for(int x=0;x<(w);x++){cout<<(m)[x][y]<<" ";}cout<<endl;}
#define ALL(v) (v).begin(),(v).end()
#define BIGINT 0x7FFFFFFF
#define E107 1000000007

template<typename T1,typename T2>
ostream& operator <<(ostream &o,const pair<T1,T2> p){o<<"("<<p.first<<":"<<p.second<<")";return o;}

int m,n,kei;
map<int,vector<pair<int,int>>> edge;

map<int,bool> place[2];

int main(){
    int i,j,k,l;
    
    cin>>n>>m>>kei;
    
    for (i=0;i<m;i++){
        scanf("%d%d%d",&j,&k,&l);
        edge[l].emplace_back(j,k);
    }
    
    cin>>k;
    for (auto e:edge[k]){
        place[0][e.first]=place[0][e.second]=true;
    }
    
    for (i=1;i<kei;i++){
        scanf("%d",&k);
        vector<pair<int,int>> &path=edge[k];
        for(auto p:place[~i&1]){
            for (auto e:path){
                if (p.first==e.first){
                    place[i&1][e.second]=true;
                }
                if (p.first==e.second){
                    place[i&1][e.first]=true;
                }
            }
        }
        place[~i&1].clear();
    }
    
    map<int,bool> &result=place[~i&1];
    printf("%d\n",result.size());
    for (auto p:result){
        printf("%d ",p.first);
    }
    cout<<endl;

    return 0;
}