#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
 
using namespace std;
using namespace __gnu_pbds;
 
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<ll,ll> pll;
typedef pair<ld,ld> pld;
typedef vector<ll> vll;
typedef vector<ld> vld;
typedef vector<pll> vpll;
typedef vector<pld> vpld;
 
#define int ll
#define all(it) it.begin(),it.end()
#define ord_set(T) tree<T,null_type,less<T>,rb_tree_tag,tree_order_statistics_node_update> 


void work(){
    int n,k,d;
    cin >> n >> k >> d;

    vll v(n);
    for (auto &c : v) cin >> c;

    vll pref(n+1);
    for (int i=0;i<n;i++) pref[i+1] = pref[i] + v[i];

    pll best{-1,n+1};
    for (int i=k;i<=n;i++) if ((pref[i] - pref[i-k])%d == 0) best = max(best,{pref[i] - pref[i-k] == k,i-k+1});
    
    if (best.second == n+1) cout << "-1\n";
    else cout << best.second << '\n';
}


int32_t main(){
    cin.tie(NULL);
    ios_base::sync_with_stdio(false);

    int n;
    cin >> n;
    while (n--) work();
    
    return 0;
}