#define MAX_NUM_WORDS 3000
#define MAX_LENGTH_WORDS 200

int
matching_count(char word[][MAX_LENGTH_WORDS], int numOfwords, int maxlenOfwords, char *key, int keylen)
{
int count = 0;

if (maxlenOfwords < keylen)
    return -1;

for (int i = 0; i < numOfwords; i++) {
    if (strlen(word[i]) < keylen)
        continue;

    if (!strncmp(word[i], key, keylen))
        count++;
}

return count;

}

void
printSubstr(char *str, int nth)
{
for (int i = 0; i < nth; i++)
printf("%c", str[i]);
printf("\n");
}

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

while(T--) {
    int     numOfwords;
    int     maxlenOfwords;
    char    words[MAX_NUM_WORDS][MAX_LENGTH_WORDS];
    char    *result;

    scanf("%d %d", &numOfwords, &maxlenOfwords);
    for (int i = 0; i < numOfwords; i++)
        scanf("%s", &words[i]);


    result = words[0];
    for (int i = 1; i <= maxlenOfwords; i++) {
        int count = -1;
        int nth = -1;
        int tmp;

        for (int j = 0; j < numOfwords; j++) {
            if (nth == i && !strncmp(words[j], result, i))
                continue;

            int strLen = strlen(words[j]);
            if (strLen > maxlenOfwords)
                break;
            else if ((tmp = matching_count(words, numOfwords, maxlenOfwords, words[j], i)) > count) {
                result = words[j];
                nth = i;
                count = tmp;
            } else if (tmp == count) {
                if (strncmp(result, words[j], i) > 0)
                    result = words[j];
            }
        }

        printSubstr(result, nth);
    }
}

return 0;

}