#include<bits/stdc++.h>
        using namespace std;
        #define endl '\n'
        #define ll long long int
        #define vv vector<long long>
        #define dq deque<long long>
        #define mpp map<long long,long long>
        #define ms multiset<long long>
        #define ss set<long long>
        #define vpp vector<pair<long long, long long>>
        #define MOD 1000000007
        #define YES cout<<"YES"<<endl
        #define NO cout<<"NO"<<endl
        #define all(a) a.begin(),a.end()
        #define sort_rev(a) sort(a.rbegin(),a.rend())
        #define all_n for(int i=0;i<n;i++)
        #define print(a) for(auto it:a)cout<<it<<" "
        #define print_pair(a) for(auto it:a)cout<<it.first<<" "<<it.second<<endl
        #define scan(a,n) for(int i=0;i<n;i++)cin>>a[i]
        void requisite(){
            ios::sync_with_stdio(false);
            cin.tie(nullptr);cout.tie(nullptr);
        }
        void solve(){
           ll n,k,d;cin>>n>>k>>d;
           vv v(n);scan(v,n);
           ll sum=0;
           int i=0,j=0;
            while (j<n){
                ll size=j-i+1;
                sum+=v[j];
    //            cout<<sum<<endl;
    //            cout<<i<<" "<<j<<endl;
                if (size==k and sum%d==0){
                    cout<<i+1<<endl;
                    return;
                }
                if (size<k){
                    j++;
                }
                else{
                    j++;
                    sum-=v[i];
                    i++;
                }
            }
            cout<<-1<<endl;
        }
        int main() {
            requisite();
            ll t=1;
            cin>>t;
            while (t--){
                solve();
            }
            return 0;
        }