#include<stdio.h>

 #include<algorithm>

 #include<string.h>

 #include<stack>

 using namespace std;

 #define MAXN 5010

 #define MAXN1 30010

 int head[MAXN],dfn[MAXN],low[MAXN],pa[MAXN],cou[MAXN],in[MAXN],ans[MAXN],s1[MAXN];

 bool vis[MAXN];

 int k,num,cnt;

 struct edg

 {

     int next,to,fr;

 }x[MAXN1];

 void add(int u,int v)

 {

     x[cnt].next=head[u];

     x[cnt].fr=u;

     x[cnt].to=v;

     head[u]=cnt++;

 }

 stack<int>s;

 void  dfs(int u)

 {

     dfn[u]=low[u]=k++;

     vis[u]=;

     s.push(u);

     int i;

     for(i=head[u];i!=-;i=x[i].next)

     {

         int t=x[i].to;

         if(!dfn[t])

         {

             dfs(t);

             low[u]=min(low[u],low[t]);

         }

         else if(vis[t])

             low[u]=min(low[u],dfn[t]);

     }

     if(dfn[u]==low[u])

     {

         num++;

         while(!s.empty())

         {

             int now=s.top();

             s.pop();

             pa[now]=num;

             vis[now]=;

             cou[num]++;

             if(now==u)break;

         }

     }

 }

 int sum;

 void dfs1(int u)

 {

     vis[u]=;

     int i;

     sum=sum+cou[u];

     for(i=head[u];i!=-;i=x[i].next)

     {

         if(!vis[x[i].to])

             dfs1(x[i].to);

     }

 }

 int main()

 {

     int t,ca;

     scanf("%d",&t);

     ca=;

     while(t--)

     {

         int n,m,i;

         scanf("%d%d",&n,&m);

         memset(head,-,sizeof(head));

         cnt=;

         for(i=;i<=m;i++)

         {

             int a,b;

             scanf("%d%d",&a,&b);

             add(a,b);

         }

         num=;

         k=;

         memset(dfn,,sizeof(dfn));

         memset(low,,sizeof(low));

         memset(vis,,sizeof(vis));

         memset(pa ,,sizeof(pa));

         memset(cou,,sizeof(cou));

         memset(in, ,sizeof(in));

         memset(ans,,sizeof(ans));

         for(i=;i<n;i++)

             if(!dfn[i])

                 dfs(i);

         int en=cnt;

         memset(head,-,sizeof(head));

         cnt=;

         for(i=;i<en;i++)

         {

             int u,v;

             u=pa[x[i].fr];

             v=pa[x[i].to];

             if(u!=v)

             {

                 add(v,u);

                 in[u]++;

             }

         }

         int m1=-;

         for(i=;i<=num;i++)

         {

             if(in[i]==)

             {

                 sum=;

                 memset(vis,,sizeof(vis));

                 dfs1(i);

                 ans[i]=sum;

                 if(sum>m1)

                     m1=sum;

             }

         }

         printf("Case %d: %d\n",ca++,m1-);

         int c1=;

         for(i=;i<n;i++)

         {

             if(ans[pa[i]]==m1)

             {

                 s1[c1++]=i;

             }

         }

         for(i=;i<c1-;i++)

             printf("%d ",s1[i]);

         printf("%d\n",s1[i]);

     }

     return ;

 }