#define _GLIBCXX_FILESYSTEM
#include<bits/stdc++.h>
using namespace std;
#define ll long long
string s[1005];
void solve() {
    int n,k;
    cin >> n >> k;
    for(int i = 0; i < n; i++) {
        cin >> s[i];
    }
    string ans;
    for(int i = 0; i+k < n; i++) {
        string now;
        for(int j = i; j <= i+k; j++)
            now += s[j];
        ans = max(ans,now);
    }
    cout << ans << '\n';
    return;
}

int32_t main() {
    ios_base::sync_with_stdio(false);cin.tie(NULL);
    int tc = 1;
    cin >> tc;
    for(int kase = 1; kase <= tc; kase++) {
        //cout << "Case " << kase << ": ";
        solve();
    }
    return 0;
}