Please explain why explicitly

For the following question, consider the following program that uses the POSIX API including its barrier implementation: pthread_barrier_t barrier; void *A(void*) { printf ("A1 "); pthread_barrier_wait(&barrier); printf("A2 "); pthread_barrier_wait(&barrier); printf("A3 "); return NULL; } void *B(void*) { for (int i = 0; i