#include<bits/stdc++.h>
#define endl        '\n'
#define F           first
#define S           second
#define pb          push_back
#define yes         cout<<"YES\n"
#define no          cout<<"NO\n"
#define all(x)      x.begin(),x.end()
#define allr(x)     x.rbegin(),x.rend()
#define error1(x)   cerr<<#x<<" = "<<(x)<<endl
#define error2(a,b) cerr<<"("<<#a<<", "<<#b<<") = ("<<(a)<<", "<<(b)<<")\n";
#define coutall(v)  for(auto &it: v) cout << it << " "; cout << endl;
using namespace std;
using ll = long long;
using ld = long double;

void solve() {
    ll n, m, k, c = 0, ans = 0;
    string s1, s2;
    cin >> n >> k;
    vector<string> v(n);
    for (int i = 0; i < n; i++) {
        cin >> v[i];
    }
    s1 = "";
    for(int i = 0; i + k + 1 <= n; i++) {
        s2 = "";
        for(int j = i; j <= i + k; j++) {
            s2 += v[j];
        }
        s1 = max(s1, s2);
    }
    cout << s1 << endl;
    return;
}

signed main() {
    ios::sync_with_stdio(false); cin.tie(0);
    int tc = 1;
    cin >> tc;
    for (int t = 1; t <= tc; t++) {
        // cout << "Case " << t << ": ";
        solve();
    }
    return 0;
}