#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k;
  cin >> n >> k;
  vector<int> row;
  vector<vector<int>> g(n, row);
  vector<vector<int>> tree(n, row);
  for (int i = 0; i < n - 1; i++) {
    int u, v;
    cin >> u >> v;
    u--;
    v--;
    g[u].push_back(v);
    g[v].push_back(u);
  }
  vector<int> ht(n, 0);
  for (int i = 0; i < 2 * k; i++) {
    int inp;
    cin >> inp;
    inp--;
    ht[inp] = 1;
  }
  queue<int> q;
  vector<int> vis(n, 0);
  vector<int> lot;
  q.push(0);
  vis[0] = 1;
  lot.push_back(0);
  while (!q.empty()) {
    int curr = q.front();
    q.pop();
    for (int j = 0; j < g[curr].size(); j++) {
      int vtx = g[curr][j];
      if (!vis[vtx]) {
        vis[vtx] = 1;
        tree[curr].push_back(vtx);
        q.push(vtx);
        lot.push_back(vtx);
      }
    }
  }
  vector<int> dp(n, 0);
  for (int i = n - 1; i >= 0; i--) {
    int curr = lot[i];
    if (ht[curr]) dp[curr]++;
    for (int j = 0; j < tree[curr].size(); j++) dp[curr] += dp[tree[curr][j]];
  }
  int root = 0;
  while (1) {
    int newroot = -1;
    for (int i = 0; i < tree[root].size(); i++) {
      int vtx = tree[root][i];
      if (dp[vtx] > k) {
        newroot = vtx;
        break;
      }
    }
    if (newroot > -1)
      root = newroot;
    else
      break;
  }
  vector<int> pairs;
  vector<int> vis2(n, 0);
  vis2[root] = 1;
  for (int i = 0; i < tree[root].size(); i++) {
    int st = tree[root][i];
    q.push(st);
    vis2[st] = 1;
    while (!q.empty()) {
      int curr = q.front();
      if (ht[curr]) pairs.push_back(curr);
      q.pop();
      for (int j = 0; j < g[curr].size(); j++) {
        int vtx = g[curr][j];
        if (!vis2[vtx]) {
          vis2[vtx] = 1;
          q.push(vtx);
        }
      }
    }
  }
  for (int i = 0; i < n; i++)
    if (ht[i] && !vis2[i]) pairs.push_back(i);
  if (ht[root]) pairs.push_back(root);
  cout << 1 << endl << root + 1 << endl;
  for (int i = 0; i < k; i++)
    cout << pairs[i] + 1 << " " << pairs[i + k] + 1 << " " << root + 1 << endl;
  return 0;
}
