#include <iostream>

using namespace std;

int N;
int M;
int ADJ[26][26];
bool VISITED[26];
int ORDER[26];
int COUNT;

void dfs(int here)
{
    VISITED[here] = true;
    for (int there = N - 1; there >= 0; there--) {
        if (ADJ[here][there] && !VISITED[there]) {
            dfs(there);
        }
    }
    ORDER[COUNT++] = here;
}

void makeGraph()
{
    for (int i = 0; i < M; i++) {
        char first, second;
        if (N == 1) {
            cin >> first;
            second = first;
        } else {
            cin >> first >> second;
        }
        ADJ[first - 'A'][second - 'A'] = 1;
    }
}

void dfsAll()
{
    for (int i = N - 1; i >= 0; i--) {
        if (!VISITED[i])
            dfs(i);
    }
}

void init()
{
    COUNT = 0;
    for (int i = 0; i < 26; i++)
        for (int j = 0; j < 26; j++)
            ADJ[i][j] = 0;
    for (int i = 0; i < 26; i++) {
        VISITED[i] = false;
        ORDER[i] = -1;
    }
}

int main(int c, char **v)
{
    ios_base::sync_with_stdio();
    int C;
    cin >> C;
    while (C--) {
        cin >> N >> M;
        init();
        makeGraph();
        dfsAll();
        for (int i = COUNT - 1; i >= 0; i--)
            cout << (char)(ORDER[i] + 'A');
        cout << endl;
    }
    return 0;
}