/* Problem URL: https://codeforces.com/contest/2194/problem/C */

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;

template <class T, class comp = less<>>
using ordered_set = tree<T, null_type , comp , rb_tree_tag , tree_order_statistics_node_update>;

#define V vector

#define rmin(a, b) a = min(a, b)
#define rmax(a, b) a = max(a, b)

#define rep(i, lim) for (int i = 0; i < (lim); i++)
#define nrep(i, s, lim) for (int i = s; i < (lim); i++)

#define repv(i, v) for (auto &i : (v))
#define fillv(v) for (auto &itr_ : (v)) { cin >> itr_; }
#define sortv(v) sort(v.begin(), v.end())
#define all(v) (v).begin(), (v).end()

using vi = vector<int>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using vvvvi = vector<vvvi>;

using ll = long long;

using vl = vector<ll>;
using vvl = vector<vl>;
using vvvl = vector<vvl>;
using vvvvl = vector<vvvl>;

template<class v>
auto operator<<(ostream &os, const vector<v> &vec)->ostream& {
    os << vec[0];
    for (size_t i = 1; i < vec.size(); i++) {
        os << ' ' << vec[i];
    }
    os << '\n';
    return os;
}

template<class v>
auto operator>>(istream &is, vector<v> &vec)->istream& {
    for (auto &i : vec) {
        is >> i;
    }
    return is;
}

template<class v>
auto operator<<(ostream &os, const vector<vector<v>> &vec)->ostream& {
    for (auto &i : vec) {
        os << i[0];
        for (size_t j = 1; j < i.size(); j++) {
            os << ' ' << i[j];
        }
        os << '\n';
    }
    return os;
}

template<class v>
auto operator>>(istream  &is, vector<vector<v>> &vec)->istream& {
    for (auto &i : vec) {
        for (auto &j : i) {
            is >> j;
        }
    }
    return is;
}

const int oo = INT32_MAX >> 1;
const ll OO = INT64_MAX >> 1;


void pre()
{

}

#define TEST 1

void solve()
{
    int n, k;
    cin >> n >> k;

    bitset<26> ev;
    rep(i, 26) {
        ev[i] = true;
    }

    V<string> a(k);
    cin >> a;

    vi divs;
    vi tp;

    for (int i = 1; i * i <= n; i++) {
        if (n % i == 0) {
            divs.push_back(i);
            if (i * i != n) {
                tp.push_back(n / i);
            }
        }
    }
    for (int i = tp.size() - 1; i >= 0; i--) {
        divs.push_back(tp[i]);
    }

    string ans(n, ' ');

    auto check = [&](int sz) {
        rep(i, sz) {
            bitset<26> val = ev;

            for (int j = i; j < n; j += sz) {
                bitset<26> cur;
                rep(l, k) {
                    cur[a[l][j] - 'a'] = true;
                }
                val &= cur;
                if (val == 0) {
                    return false;
                }
            }

            char ch = 63 - __builtin_clzll(val.to_ullong()) + 'a';
            for (int j = i; j < n; j += sz) {
                ans[j] = ch;
            }
        }

        return true;
    };

    repv(i, divs) {
        if (check(i)) {
            cout << ans << '\n';
            return;
        }
    }

    assert(false);
}

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    pre();

    int t;
    (TEST && cin >> t) || (t = 1);
    while (t--) {
        solve();
    }
}