#include<bits/stdc++.h>
using namespace std;
#define int long long
signed main(){
    int t;
    cin >> t;
    while(t--){
        int n, k;
        cin >> n >> k;
        string ans = "0", tmp;
        string s[n];
        int sz[n];
        for(int i = 0; i < n; i++){
            cin >> s[i];
            tmp += s[i];
            int id = i - k - 1;
            sz[i] = s[i].size();
            if(id >= 0) tmp = tmp.substr(sz[id], tmp.size() - (sz[id]));
            ans = max(ans, tmp);
        }
        cout << ans << "\n";
    }
    return 0;
}