#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, k;
string s1, s2;
cin >> n >> k;
cin >> ws;
vector<string> v(n);
for (int i = 0; i < n; i++) {
cin >> v[i];
}
s1 = "";
k += 1;
for(int i = 0; i + k <= n; i++) {
s2 = "";
for(int j = i; j < i + k; j++) {
s2 += v[j];
}
// cout << i << " " << i + k -1 << " " << s2 << endl;;
if(s1 < s2) 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;
}