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

int main() {
    int T;
    cin >> T;

    while (T--) {
        int N, K;
        cin >> N >> K;

        vector<string> numbers(N);
        for (int i = 0; i < N; i++) {
            cin >> numbers[i];
        }

        sort(numbers.begin(), numbers.end(), [](const string& a, const string& b) {
            return a.length() > b.length() || (a.length() == b.length() && a > b);
        });

        for (int i = 0; i < K; i++) {
            numbers[i] += numbers[i + 1];
            numbers.erase(numbers.begin() + i + 1);
        }

        cout << numbers[0] << endl;
    }

    return 0;
}