#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
// #define cerr cout
#include "algo/debug.h"
#else
#define deb(...) 30
#endif
#define ll long long
#define all(x) (x).begin(), (x).end()
#define f(i, n) for (int i = 0; i < n; i++)
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL);
int t;
cin >> t;
while (t--)
{
int n, k;
cin >> n >> k;
vector<string> v(n);
for (auto &a : v)
cin >> a;
vector<string> final;
for (int i = 0; i < n - k; i++)
{
string temp = "";
for (int j = 0, kk = i; j <= k; j++, kk++)
{
temp += v[kk];
}
final.push_back(temp);
}
sort(all(final));
// for (auto a : final)
// cout << a << '\n';
cout << final.back() << '\n';
}
return 0;
}