#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
// #define cerr cout
#include "algo/debug.h"
#else
#define deb(...) 30
#endif
#define ll long long
#define all(x) (x).begin(), (x).end()
#define f(i, n) for (int i = 0; i < n; i++)

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    int t;
    cin >> t;
    while (t--)
    {
        int n, k;
        cin >> n >> k;
        vector<string> v(n);
        for (auto &a : v)
            cin >> a;
        // vector<string> final;
        string final = "";
        for (int i = 0; i <= n - k - 1; i++)
        {
            string temp = "";
            for (int j = 0; j <= k; j++)
            {
                temp += v[i + j];
            }
            // cout << ".\n";
            if (temp > final)
                final = temp;
        }
        cout << final << '\n';
    }

    return 0;
}