#include <bits/stdc++.h>
#define endl '\n'
#define F first
#define S second
using namespace std;
#define ll long long
#define pb push_back
#define mod 1000000007
// #define int long long
#define all(x)      x.begin(),x.end()
#define allr(x)     x.rbegin(),x.rend()
#define CheckBit(x,k)   (x & (1LL << k))
#define SetBit(x,k)     (x |= (1LL << k))
#define ClearBit(x,k)   (x &= ~(1LL << k))
#define LSB(mask)       __builtin_ctzll(mask)
#define MSB(mask)       63-__builtin_clzll(mask) 
#define print(x)    cout << #x << " : " << x << endl
#define error1(x)   cerr << #x << " = " << (x) <<endl
#define coutall(v)  for(auto &it: v) cout<<it<<' '; cout<<endl
#define Abid_52     ios::sync_with_stdio(false);cin.tie(0),cin.tie(0)
#define error2(a,b) cerr<<"( "<<#a<<" , "<<#b<<" ) = ( "<<(a)<<" , "<<(b)<<" )\n"
#define UNIQUE(x)   sort(all(x)), x.erase(unique(all(x)), x.end()), x.shrink_to_fit()
template <typename T, typename U> T ceil(T x, U y) {return (x > 0 ? (x + y - 1) / y : x / y);}
template <typename T, typename U> T floor(T x, U y) {return (x > 0 ? x / y : (x - y + 1) / y);}

const int N = 2e5 + 10;

void solve()
{
    int n, m;
    cin >> n >> m;
    vector<string> v(n);
    for (int i = 0; i < n; i++)
    {
        cin >> v[i];
    }
    vector<string>temp;
    int mx = -1;
    for(int i = 0; i + m < n; i++){
        string x;
        for(int j = i; j <= i + m; j++){
            x += v[j];
        }
        temp.push_back(x);
        mx = max(mx,(int)x.length());
    }
    vector<string>ans;
    string fans;
    for(int i = 0; i < temp.size(); i++){
        if(temp[i].length() == mx){
            if(fans.empty()){
                fans = temp[i];
            }
            else{
                for(int j = 0; j < fans.length(); j++){
                    if(fans[j] < temp[i][j]){
                        fans = temp[i];
                        break;
                    }
                    else if(fans[j] > temp[i][j])break;
                }
            }
        }
    }
    cout << fans << endl;

}

int32_t main()
{
    Abid_52;
    int t = 1;
    cin >> t;
    for (int tc = 1; tc <= t; ++tc)
    {
        // cout << "Case " << tc << ": ";
        solve();
    }
}