using i64 = long long;
using i128 = __int128;
using u32 = unsigned;
using u64 = unsigned long long;
using f32 = double;
using f64 = long double;

#define uset unordered_set
#define umap unordered_map
#define vi vector<int>
#define vvi vector<vi>
#define vll vector<i64>
#define vvll vector<vll>
#define pii pair<int, int>
#define pll pair<i64, i64>
#define vpii vector<pii>
#define vpll vector<pll>
#define vvpii vector<vpii>
#define vvpll vector<vpll>
#define vz vector<Z>
#define vvz vector<vz>
#define pb push_back
#define pq priority_queue
#define ALL(x) (x).begin(), (x).end()
#define rep(i, x, y) for (int (i) = (x); (i) < (y); (i)++)
#define repr(i, x, y) for (int (i) = (x); (i) > (y); (i)--)
#define YES "YES\n"
#define NO "NO\n"
#define SZ(x) (static_cast<int>(x.size()))


#include <bits/stdc++.h>

using namespace std;

mt19937_64 rng((unsigned) chrono::high_resolution_clock::now().time_since_epoch().count());

void solve() {
    int n, k, d;
    cin >> n >> k >> d;
    vi a(n);
    rep(i, 0, n) cin >> a[i];
    vi pre(n + 1);
    rep(i, 0, n) pre[i + 1] = pre[i] + a[i];
    int mx = -1, id = -1;
    rep(i, 0, n - k + 1) {
        if ((pre[i + k] - pre[i]) % d == 0) {
            int f = 0;
            if (pre[i + k] - pre[i] == k) f = 1;
            if (f > mx) {
                mx = f;
                id = i + 1;
            }
        }
    }
    cout << id << "\n";
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);

    int t;
    cin >> t;
    while (t--) solve();
}