#include "bits/stdc++.h"
using namespace std;
using ll = long long;
#define int ll
using pii = pair<int, int>;
template<typename T> using minHeap = priority_queue<T, vector<T>, greater<T>>;
#define all(x) x.begin(),x.end()
#define V vector
#define size(v) (int(v.size()))
const ll mxn= 2e5+3, inf= 1.5e18,mod= 1e9+7;//998244353;
// iwkms
void pre(){}
void solve() {
    int n, k, d;cin>>n>>k>>d;
    V<int> a(n+5), pre=a;
    for (int i=1;i<=n;i++)cin>>a[i] , pre[i] = a[i] + pre[i-1];
    int ans = -1, prod = -1;
    for (int i=k;i<=n;i++) {
        int sm = pre[i] - pre[i-k];
        if (sm %d == 0) {
            int pr = sm==k;
            if (pr > prod) prod = pr, ans = i-k+1;
        }
    }
    cout<<ans<<"\n";
}
int32_t main() 
{
    pre();
    cin.tie(0)->sync_with_stdio(0);
    int t=1;cin>>t;while(t--)
        solve();
    return 0;
}