#include<bits/stdc++.h>
using namespace std;
#define int long long

const int N = 3e5 + 9;
const int inf = 3e10 + 9;

int a[N], b[N];
int dp[1003][103];

string s[N];

vector<int> v[7];

std::map<string, int> mp;

int32_t main() {

    int tc = 1;
    cin >> tc;
    while(tc--) {
        int n, k; cin >> n >> k;
        for (int i = 0; i < n; ++i)
        {
            cin >> s[i];
        }
        string ans = "";
        for(int i = 0; i < n; i++)
        {
            string tmp = "";
            for(int j = i; j <= i + k; j++)
            {
                tmp += s[j];
            }
            ans = max(ans, tmp);
        }
        cout << ans << endl;
    }

    return 0;
}