what is the length of s1 after line 4 is executed?
char s1[21];
char s2[50] = "got your tongue?";
char s3[10] = "Cat";
strncpy(s1, s2, sizeof(s1) - 1);
strncat(s1, s3, sizeof(s1) - strlen(s1) - 1);
printf("%s", s1);