#include <bits/stdc++.h>

#ifdef LOCAL
#include "template.cpp.h"
#else
#define debug(...)
#endif

#define int long long
using namespace std;
#define cinv(v) for (auto &it:v) cin>>it;
#define coutv(v) for (auto &it:v) cout<< it<<' '; cout<<'\n';

string max(string &a, string &b) {
    if (a.size() != b.size()) return (a.size() > b.size() ? a : b);
    for (int i = 0; i < a.size(); ++i) {
        if (a[i] != b[i]) {
            return (a[i] > b[i] ? a : b);
        }
    }
    return a;
}

void shelby() {
    int n, k;
    cin >> n >> k;
    vector<string> v(n);
    cinv(v)
    string ans;
    for (int i = 0; i < n; ++i) {
        string tmp;
        for (int j = i, cnt = k; j < n && cnt >= 0; ++j, --cnt) tmp += v[j];
        ans = max(ans, tmp);
    }
    cout << ans << '\n';
}

signed main() {
    cin.tie(0)->ios_base::sync_with_stdio(0);
    int t = 1;
    cin >> t;
    for (int _ = 1; _ <= t; ++_) {
//        cout << "Case " << _ << ": ";
        shelby();
    }
}