#include<bits/stdc++.h>
using namespace std;
#define int long long int 
#define nl "\n"
#define all(x) (x).begin(), (x).end()
#define Yes cout<<"Yes"<<"\n";
#define No  cout<<"No"<<"\n"; 
string concatenate(const string &a, const string &b) {
    return a + b;
}
struct Compare {
    bool operator()(const pair<string, int> &a, const pair<string, int> &b) {
        return a.first < b.first;
    }
};
void run(int t){
    int N, K;
        cin >> N >> K;
        vector<string> a(N);
        for (int i = 0; i < N; ++i) {
            cin >> a[i];
        }
        priority_queue<pair<string, int>, vector<pair<string, int>>, Compare> maxHeap;
        for (int i = 0; i < N - 1; ++i) {
            string concat1 = concatenate(a[i], a[i + 1]);
            maxHeap.push({concat1, i});
        }
        for (int i = 0; i < K; ++i) {
            if (maxHeap.empty()) break;

            auto [maxConcat, index] = maxHeap.top();
            maxHeap.pop();

            a[index] = maxConcat;
            a.erase(a.begin() + index + 1);

            if (index > 0) {
                string concat1 = concatenate(a[index - 1], a[index]);
                maxHeap.push({concat1, index - 1});
            }
            if (index < a.size() - 1) {
                string concat2 = concatenate(a[index], a[index + 1]);
                maxHeap.push({concat2, index});
            }
        }
        cout << *max_element(all(a)) << nl;
}
int32_t main(){
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    int t=1;
    cin >> t;
    int c  = 1;
    while(t--){
        //cout << "Case " << c++ << ": ";
        run(t);
    }
    return 0;
}