#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
#include "algo/debug.h"
#else
#define debug(...) 42
#endif
int main() {
ios::sync_with_stdio(false);
cin.tie(0);
int t;
cin >> t;
while(t--) {
int n,k;
cin >> n >> k;
vector<string> a(n);
for(int i=0;i<n;i++){
cin >> a[i];
}
deque<string> ass;
string nn="0";
string ans;
for(int i=0;i<k+1;i++){
ass.push_back(a[i]);
ans+=a[i];
}
nn = max(nn,ans);
ans.clear();
for(int i=k+1;i<n;i++){
ass.pop_front();
ass.push_back(a[i]);
for(auto as:ass){
ans+=as;
}
nn = max(nn,ans);
ans.clear();
}
cout << nn <<"\n";
// m = max(m,s);
// cout << m <<"\n";
}
}