//on the name of Allah:)
#include<bits/stdc++.h>
#define int         long long
#define Endl        "\n"
#define pi          2 * acos(0.0)
#define mod         1000000007
#define Mul(a,b)    (a%mod * b%mod)%mod
#define Add(a,b)    (a%mod + b%mod)%mod
#define all(x)      (x).begin(),(x).end()
#define allr(x)     (x).rbegin(),(x).rend()
#define gcd(x, y)   (__gcd(x, y))
#define lcm(x, y)   ((x/gcd(x, y))*y)
#define faster      cin.tie(NULL), cout.tie(NULL);
#define TC          int t ; cin>>t ; for(int i=1;i<=t;i++)
const int N = 1e6 + 7;
using namespace std;
void s()
{
   int n,k;
   cin>>n>>k;
   vector<string>v(n);
   for(int i=0;i<n;i++)cin>>v[i];



    while (k > 0 && v.size() > 1) {
        string mx = "";
        int idx = 0;

        for (int i = 0; i < n - 1; i++) {
            string c1 = v[i] + v[i + 1];
            string c2 = v[i + 1] +v[i];
            if (c1 > c2 && c1 > mx) {
                mx = c1;
                idx = i;
            } else if (c2 > c1 && c2 > mx) {
                mx = c2;
                idx = i + 1;
            }
        }

        if (idx > 0 && (v[idx - 1] + v[idx]) == mx) {
            v[idx - 1] = mx;
          v.erase(v.begin() + idx);
        } else {
            v[idx] = mx;
            v.erase(v.begin() + idx+1);
        }


        --n;
        --k;
    }
    string ans=*max_element(all(v));
    cout<<ans<<endl;



    }



int32_t main()
{
    ios::sync_with_stdio(false);
    //faster
    TC
    {
        //cout<<"Case "<<i<<": ";
        s();

    }



}