#include <bits/stdc++.h>
using namespace std;

string solve(int n, int k, vector<string>& arr) {
    priority_queue<pair<string, int>> pq;

    for (int i=0; i<n-1; i++){
        string concat=arr[i]+arr[i+1];
        pq.push({concat,i});
    }

    
    for (int i=0; i<k && !pq.empty(); i++) {
        auto top=pq.top();
        pq.pop();

        string max_con = top.first;
        int pos = top.second;

        arr[pos] = max_con;
        arr.erase(arr.begin()+pos+1);

        if (pos>0) {
            string concat =arr[pos-1]+arr[pos];
            pq.push({concat,pos-1});
        }
        if (pos<arr.size()-1) {
            string concat=arr[pos]+arr[pos+1];
            pq.push({concat,pos});
        }
        --n; 
    }

    return *max_element(arr.begin(), arr.end());
}

int main() {
    int t;
    cin>>t;

    while(t--) {
        int n,k;
        cin>>n>>k;
        vector<string> str(n);

        for(int i=0; i<n; i++) {
            cin>>str[i];
        }

        cout << solve(n, k, str) << endl;
    }

    return 0;
}