#include <bits/stdc++.h>
using namespace std;
const int MAXN = 1e5 + 10;
const long long MOD = 1e9 + 7;
int n, m, kk, x;
vector<int> g[MAXN];
int sz[MAXN];
long long dp[MAXN][3][11];
long long tmp[3][11];
long long c[3];
void update(long long &a, long long b, long long cc, int d) {
  a = (a + (cc * b % MOD)) % MOD;
}
void dfs(int u, int fa) {
  sz[u] = 1;
  dp[u][0][0] = dp[u][2][0] = dp[u][1][1] = 1;
  for (int i = 0; i < g[u].size(); i++) {
    int v = g[u][i];
    if (v == fa) continue;
    dfs(v, u);
    memset(tmp, 0, sizeof(tmp));
    for (int k = min(sz[v], x); k >= 0; k--) {
      for (int j = 0; j < 3; j++)
        if (dp[v][j][k]) {
          for (int p = x - k; p >= 0; p--) {
            update(tmp[0][p + k], dp[u][0][p], dp[v][j][k], 0);
            if (j == 0) {
              update(tmp[1][p + k], dp[u][1][p], dp[v][j][k], 1);
              update(tmp[2][p + k], dp[u][2][p], dp[v][j][k], 2);
            } else if (j == 2)
              update(tmp[2][p + k], dp[u][2][p], dp[v][j][k], 2);
          }
        }
    }
    memcpy(dp[u], tmp, sizeof(tmp));
    sz[u] += sz[v];
  }
  for (int i = 0; i < 3; i++)
    for (int j = 0; j <= x; j++) dp[u][i][j] = dp[u][i][j] * c[i] % MOD;
}
int main() {
  cin >> n >> m;
  for (int i = 1; i < n; i++) {
    int u, v;
    scanf("%d%d", &u, &v);
    g[u].push_back(v);
    g[v].push_back(u);
  }
  cin >> kk >> x;
  c[0] = kk - 1;
  c[1] = 1;
  c[2] = m - kk;
  dfs(1, 0);
  long long ans = 0;
  for (int i = 0; i < 3; i++)
    for (int j = 0; j <= x; j++) {
      ans = (ans + dp[1][i][j]) % MOD;
    }
  cout << ans << endl;
}
