Handling Multiple wait operation before entering into critical section in operating system?

Question

I am stuck while solving a counting semaphore problem in Operating system subject.

S is a semaphore initialized to 5.

count = 0 (shared variable) 

Assume that the increment operation in line #7 is not atomic.

Now,

1.  int counter = 0;
2.  Semaphore S = init(5);
3.  void parop(void)
4.  {
5.         wait(S);
6.         wait(S);
7.         counter++;
8.         signal(S);
9.         signal(S);
10.  }

If five threads execute the function parop concurrently, which of the following program behavior(s) is/are possible?

A. The value of counter is 5 after all the threads successfully complete the execution of parop

B. The value of counter is 1 after all the threads successfully complete the execution of parop

C. The value of counter is 0 after all the threads successfully complete the execution of parop

D. There is a deadlock involving all the threads

what i have understand till now is answer is A and D,because what if all process are executed one by one say(T1->T2->T3->T4->T5) and final value saved will be 5(so A is one of correct options) Now, why D,because what if all process execute line 5 before 6 and will get blocked.

Now, please can any one help me to understand why B is another correct answer. ?

Thanks in advance,

Hope to here from you soon

Any help will be highly appreciated.