#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> max_heap;
    for (int i = 0; i < n - 1; ++i) {
        string concat1 = concatenate(a[i], a[i + 1]);
        max_heap.push({concat1, i});
    }
    for (int i = 0; i < k; ++i) {
        if (max_heap.empty()) break;

        auto [max_concat, index] = max_heap.top();
        max_heap.pop();

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

        if (index > 0) {
            string concat1 = concatenate(a[index - 1], a[index]);
            max_heap.push({concat1, index - 1});
        }
        if (index < a.size() - 1) {
            string concat2 = concatenate(a[index], a[index + 1]);
            max_heap.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;
}