#include<bits/stdc++.h>
#define ll long long
using namespace std;

int main() {
  int t;
  cin >> t;
  while (t--){
    ll n, k, d, ans = -1, mx = -1;
    cin >> n >> k >> d;
    vector<ll> a(n + 1);
    for (int i = 1; i <= n; i++){
      cin >> a[i];
      a[i] += a[i - 1];
    }
    for (int i = 1; i + k - 1 <= n; i++){
      if ((a[i + k - 1] - a[i - 1]) % d == 0 && (a[i + k - 1] - a[i - 1]) > mx){
        if (mx < 0) ans = i;
        mx = (a[i + k - 1] - a[i - 1]);
        if ((a[i + k - 1] - a[i - 1]) == k) {
          ans = i;
          break;
        }
      }
    }
    cout << ans << endl;
  }
}