Question 4: Suppose that the following two processes/threads are run on a single-CPU system. The code below is informal, but the meaning of all statements is clear. The semantics of test_and_set is as defined in class. /* These variables are shared by both threads */ int guard =1, sum =0; What is the value of sum printed by P2 before the thread terminates? a. 0 b. 2 c. 4 d. 6 e. 8 f. 10 g. 12 h. 14