#define SIZE 27
#define CONNECTSIZE 101
int TC;
int N;
int M;
char Input[SIZE][2] = { 0, };
int Data[SIZE][CONNECTSIZE] = { 0, };
int DataOut[SIZE] = { 0, };
int DataIn[SIZE] = { 0, };
int Que[SIZE];
int Header = 0, Tail = 0;


void PRINT_DATA()
{
    for (int i = 0; i < N; i++)
    {
        printf("%3d", DataOut[i]);
    }
    printf("\n");
    for (int i = 0; i < N; i++)
    {
        printf("%3d", DataIn[i]);
    }
    printf("\n");

}

void PRINT_QUE()
{
    //printf("[QUE] ");
    for (int i = 0; i < Tail; i++)
    {
        printf("%c", Que[i]+65);
    }
    printf("\n");
}

void Clean()
{
    Header = 0;
    Tail = 0;
    for (int i = 0; i < N; i++)
    {
        DataOut[i] = 0;
        DataIn[i] = 0;
        Que[i] = 0;
    }
}

void AddQue(int Value)
{
    int Temp;
    Que[Tail++] = Value;
    //PRINT_QUE();
    for (int i = Header + 1; i < Tail; i++)
    {
        int j = i;
        while (j>Header + 1 && Que[j - 1] > Que[j])
        {
            Temp = Que[j-1];
            Que[j - 1] = Que[j];
            Que[j] = Temp;
            j--;
        }

    }
    //PRINT_QUE();

}
void Solved()
{
    for (int i = 0; i < N; i++)
    {
        if (DataIn[i] == 0)
        {
            Que[Tail++] = i;
        }

    }
    //PRINT_QUE();

    while (Header != Tail)
    {
        for (int i = 0; i < DataOut[Que[Header]]; i++)
        {
            DataIn[Data[Que[Header]][i]]--;
            if (DataIn[Data[Que[Header]][i]] == 0)
            {
                AddQue(Data[Que[Header]][i]);
                //printf("Add Que\n");
                //PRINT_QUE();
            }
        }
        Header++;
        //printf("Header:%d,Tail:%d\n", Header, Tail);
        //PRINT_QUE();
    }

}
int main(void)
{
    scanf("%d", &TC);

    for (int i = 0; i < TC; i++)
    {
        scanf("%d %d", &N, &M);
        Clean();
        for (int j = 0; j < M; j++)
        {
            scanf("%s", &Input[j]);
            Data[Input[j][0] - 'A'][DataOut[Input[j][0] - 'A']++] = Input[j][1] - 'A';
            DataIn[Input[j][1] - 'A']++;
        }

        if (M == 0)
        {
            for (int k = 65; k < 65 + N; k++)
            {
                printf("%c", k);
            }
            printf("\n");
        }
        else if (N == 1)
        {
            printf("A\n");
        }
        else
        {
            Solved();
            PRINT_QUE();
        }
    }
//  PRINT_DATA();
    //printf("%d,%d\n", Data[0][0], Input[0][1]);


    return 0;
}