#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
using namespace std;

int main() {
    int T;
    cin >> T;
    while (T--) {
        int N, K;
        cin >> N >> K;
        vector<string> arr(N);
        for (int i = 0; i < N; i++) {
            cin >> arr[i];
        }

        string maxNumber = "";
        for (int i = 0; i < N - K; i++) {
            string current = "";
            for (int j = i; j <= i + K; j++) {
                current += arr[j];
            }
            if (current > maxNumber) {
                maxNumber = current;
            }
        }

        cout << maxNumber << endl;
    }
    return 0;
}