#include <iostream>
#include <vector>
#include <algorithm>
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;
}