// I AM A MUSLIM

#include "bits/stdc++.h"

#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")

#define fastIO std::ios::sync_with_stdio(0);std::cin.tie(0)
#define ll long long int
#define flush fflush(stdout)
#define bl printf("\n")
#define yn(a, b) printf("%s\n", a >= b ? "Yes":"No")
// #define int ll

using pii = std::pair<int,int>;

const int MOD = 1000000007;
// const int MOD = 998244353;
const int mxN = 100100;

std::string calc(std::string a, std::string b) {
    if ((int)a.size() > (int)b.size()) return a;
    if ((int)a.size() < (int)b.size()) return b;
    for (int i = 0; i < (int)a.size(); i++) {
        if (a[i] > b[i]) return a;
        if (b[i] > a[i]) return b;
    }
    return a;
}

signed main() {
    // fastIO;
    int testCases=1;
    scanf("%d",&testCases);
    // std::cin>>testCases;
    
    for (int TC = 1; TC <= testCases; TC++) {
        int N, k;
        scanf("%d%d",&N,&k);
        std::vector<std::string> a(N);
        for (auto &i : a) std::cin >> i;
        
        std::string ans;
        for (int i = 0; i+k < N; i++) {
            std::string s;
            for (int j = i, c = 0; c <= k; c++,j++) {
                s += a[j];
            }
            ans = calc(ans, s);
        }
        
        printf("%s\n", ans.c_str());
        
    }
    
    return 0;
}

/*

*/