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

// Function to return the maximum of two numeric strings
string max_num(const string& a, const string& b){
    if(a.length() > b.length()) return a;
    if(a.length() < b.length()) return b;
    return (a > b) ? a : b;
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    
    int T;
    cin >> T;
    while(T--){
        int N, K;
        cin >> N >> K;
        vector<string> s(N);
        for(int i=0; i<N; i++) cin >> s[i];
        
        // Initialize max_number as the first element
        string max_number = s[0];
        for(int i=1; i<N; i++) {
            max_number = max_num(max_number, s[i]);
        }
        
        // Determine the maximum number by considering all possible concatenations
        int max_m = min(K+1, N); // Maximum group size
        for(int m=2; m<=max_m; m++){
            for(int i=0; i + m <= N; i++){
                // Concatenate elements from index i to i+m-1
                string concat = "";
                for(int j=i; j<i+m; j++) concat += s[j];
                
                // Update max_number if the concatenated string is larger
                max_number = max_num(max_number, concat);
            }
        }
        
        cout << max_number << "\n";
    }
}