#include <iostream>
#include <math.h>
#include <vector>
#include <string>
#include <algorithm>
#include <queue>
#include <stack>
#include <map>
#include <cstring>
#include <iomanip>
#include <set>
#include <bitset>
#include <unordered_map>
using namespace std;
using ll = long long;
using pii = pair<int,int>;
using piii = tuple<int,int,int>;
#define f first
#define s second
#define endl '\n'
#define all(x) begin(x),end(x)
#define m_p make_pair
int const N = 5e4+10;
int cnt[N];
class bitsegment{
struct Node{
bitset<64> val;
};
Node segment[N*4];
public:
int si;
void process(int node,int l,int r){
if(l == r) cnt[l] += segment[node].val.count();
else{
int md = l+(r-l)/2;
process(node*2,l,md);
process(node*2+1,md+1,r);
}
segment[node].val.reset();
}
void process(){
process(1,1,si);
}
void push(int node,int l,int r){
if(l == r) return;
else{
segment[node*2].val |= segment[node].val;
segment[node*2+1].val |= segment[node].val;
}
segment[node].val = 0;
}
void set(int node,int l,int r,int lay,int ql,int qr){
push(node,l,r);
if(l >= ql && r <= qr){
segment[node].val.set(lay);
push(node,l,r);
}
else if(l > qr || r < ql) return;
else{
int md = l+(r-l)/2;
set(node*2,l,md,lay,ql,qr);
set(node*2+1,md+1,r,lay,ql,qr);
}
}
void set(int l,int r,int lay){
if(l > r) return;
set(1,1,si,lay,l,r);
}
void print(int node,int l,int r){
push(node,l,r);
if(l == r){
cout << segment[node].val << endl;
}
else{
int md = l+(r-l)/2;
print(node*2,l,md);
print(node*2+1,md+1,r);
}
}
void print(){
print(1,1,si);
cout << "----------------------------------------------------------------" << endl;
}
};
bitsegment segment;
int nodecnt[N];
int depth[N];
int pos[N];
int rpos[N];
int chain[N];
int chainhead[N];
int par[N];
int curpos = 0;
int chainid = 0;
vector<int> adj[N];
void dfs1(int n,int p){
nodecnt[n] = 1;
par[n] = p;
for(auto k:adj[n]){
if(k == p) continue;
depth[k] = depth[n]+1;
dfs1(k,n);
nodecnt[n] += nodecnt[k];
}
}
void dfs2(int n,int p){
pos[n] = ++curpos;
rpos[pos[n]] = n;
chain[n] = chainid;
pii hv = {0,-1};
for(auto k:adj[n]){
if(k == p) continue;
hv = max(hv,{nodecnt[k],k});
}
if(hv.s != -1) dfs2(hv.s,n);
for(auto k:adj[n]){
if(k == p || k == hv.s) continue;
chainhead[++chainid] = k;
dfs2(k,n);
}
}
void update(int a,int b,int lay){
while(chain[a] != chain[b]){
if(depth[a] < depth[b]) swap(a,b);
int head = chainhead[chain[a]];
segment.set(pos[head],pos[a],lay);
a = par[head];
}
if(depth[a] < depth[b]) swap(a,b);
segment.set(pos[b]+1,pos[a],lay);
}
int main(){
int n;cin >> n;
int m;cin >> m;
int k;cin >> k;
segment.si = n;
for(int i{};i < n-1;i++){
int a,b;cin >> a >> b;
adj[a].emplace_back(b);
adj[b].emplace_back(a);
}
dfs1(1,1);
chainhead[0] = 1;
dfs2(1,1);
for(int i{};i < m;i++){
int g;cin >> g;
int last = 0;
if(i%64 == 0){
segment.process();
}
for(int j{};j < g;j++){
int c;cin >> c;
if(j > 0) update(last,c,i%64);
last = c;
}
}
segment.process();
vector<pii> ans;
int cnt2 = 0;
for(int i{2};i <= n;i++){
if(cnt[i] >= k){
cnt2++;
ans.emplace_back(rpos[i],par[rpos[i]]);
}
}
cout << cnt2 << endl;
sort(all(ans));
for(auto [a,b]:ans){
cout << a << " " << b << endl;
}
}
