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

const int N = 2e5+5;

int s[N];

void solve() {
    ll n,k,d;
    cin >> n >> k >> d;
    for(int i = 1; i <= n; i++) {
        int x;
        cin >> x;
        s[i] = s[i - 1] + x;
    }
    int ans = -1, mx = -1;
    for(int i = k; i <= n; i++) {
        int csum = s[i] - s[i - k];
        if(csum % d == 0) {
            int prod = 1;
            if(csum != k) prod = 0;
            if(prod > mx) {
                mx = prod;
                ans = i - k + 1;
            }
        }
    }
    cout << ans << '\n';
    return;
}

int32_t main() {
    ios_base::sync_with_stdio(false);cin.tie(NULL);
    int tc = 1;
    cin >> tc;
    for(int kase = 1; kase <= tc; kase++) {
        //cout << "Case " << kase << ": ";
        solve();
    }
    return 0;
}