#pragma GCC optimize("Ofast")
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int MOD = 1000000007;
#define sz(x) (ll)(x).size()
#define rd ({ll x; cin >> x; x; })
#define dbg(x) cerr << "[" #x "] " << (x) << "\n\n"
// #define errv(x) {cerr << "["#x"] ["; for (const auto& ___ : (x)) cerr << ___ << ", "; cerr << "]\n";}
// #define errvn(x, n) {cerr << "["#x"] ["; for (auto ___ = 0; ___ < (n); ++___) cerr << (x)[___] << ", "; cerr << "]\n";}
// #define cerr if(0)cerr
#define xx first
#define yy second
mt19937 rnd(std::chrono::high_resolution_clock::now().time_since_epoch().count());
/*_________________________________________________________________________________________________________________________*/
void Solve()
{
ll n, k;
cin >> n >> k;
vector<string> arr(n);
for (int i = 0; i < n; i++)
cin >> arr[i];
set<string> s;
for (int i = 0; i < n; i++) {
string tmp = arr[i];
ll K = k;
for (int j = i + 1; j < n && K; j++) {
tmp += arr[j];
K--;
}
s.insert(tmp);
}
cout << *s.rbegin() << '\n';
}
int32_t main()
{
ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
int t = 1;
cin >> t;
for (int i = 1; i <= t; i++) {
// cout << "Case #" << i << ": "; // cout << "Case " << i << ": ";
Solve();
}
return 0;
}
// Coded by Tahsin Arafat (@TahsinArafat)