#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <map>
#include <set>
//#include <queue>
//#include <numeric>
#include <cassert>

using namespace std;

#ifdef LOCAL_DEBUG
#include <local_debug.h>
#define DEBUG(...) DBG2::cprint(#__VA_ARGS__, __LINE__, __VA_ARGS__)
#else
#define DEBUG(...)
#endif

#define SZ(a) int((a).size())
#define REP(i,n) for(int i=0,_n=(n);i<_n;++i)
#define FOR(i,a,b) for(int i=(a),_b=(b);i<=_b;++i)

using llong = long long;
using VI = vector<int>;
using VVI = vector<VI>;
using II = pair<int,int>;

int solve(int N, VI A, int K, int D) {
   int sum = 0;
   for (int i = 0; i < K; i++)
      sum += A[i];

   int res = -1;
   int max_prod = -1;
   
   if ((sum % D) == 0) {
      max_prod = sum == K ? 1 : 0;
      res = 1;
   }

   FOR(i, K, N-1) {
      sum += A[i];
      sum -= A[i-K];

      if ((sum % D) == 0) {
         int prod = sum == K ? 1 : 0;
         if (prod > max_prod) {
            max_prod = prod;
            res = i-K+2;
         }
      }
   }
   return res;
}

int main(int argc, char* argv[]) {
   ios_base::sync_with_stdio(false); 
   cin.tie(nullptr);

   int TC;
   cin >> TC;
   FOR(tc, 1, TC) {
      int N, K, D;
      cin >> N >> K >> D;
      VI A(N);
      REP(i, N)
         cin >> A[i];
      int res = solve(N, A, K, D);
      cout << res << '\n';
   }

   return 0;
}