#include<bits/stdc++.h>
using namespace std;
const long long M=3e5+10,MOD=1000000000;
typedef long long ll;

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    int t=1;
    cin>>t;
    while(t--){
        int n,k;
        cin>>n>>k;
        vector<int>a(n+1,0);
        string p="";
        for(int i=1;i<=n;i++){
            string s;
            cin>>s;
            p+=s;
            a[i]=(int)s.size();
            a[i]+=a[i-1];
        }
        int maximum_digit=0;
        for(int i=1;i<=n;i++){// target to find maximum digit possible
            if(i+k<=n)maximum_digit=max(maximum_digit,a[i+k]-a[i-1]);
        }
        vector<int>pos;
        for(int i=1;i<=n;i++){
            if(i+k<=n&&a[i+k]-a[i-1]==maximum_digit)pos.push_back(i);
        }
        vector<string>ans;
        for(int x:pos){
            string s="";
            int l=a[x-1];
            while(s.size()<maximum_digit)s+=p[l],l++;
            ans.push_back(s);
        }
        sort(ans.begin(),ans.end());
        cout<<ans[ans.size()-1]<<"\n";


    
    }


   
   return 0;
 
}