What is the value printed by the following C program?

#include<stdio.h> int f(int *a, int n) {   if(n <= 0) return 0;   else if(*a % 2 == 0) return *a + f(a+1, n-1);   else return *a - f(a+1, n-1); }   int main() {   int a[] = {12, 7, 13, 4, 11, 6};   printf("%d", f(a, 6));   getchar();   return 0; }

(A) -9
(B) 5
(C) 15
(D) 19

Answer (C)
f() is a recursive function which adds f(a+1, n-1) to *a if *a is even. If *a is odd then f() subtracts f(a+1, n-1) from *a. See below recursion tree for execution of f(a, 6).
.

 f(add(12), 6) /*Since 12 is first element. a contains address of 12 */
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 12 + f(add(7), 5) /* Since 7 is the next element, a+1 contains address of 7 */
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     7 - f(add(13), 4)
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           13 - f(add(4), 3)
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                  4 + f(add(11), 2)
                             |
                             |
                           11 - f(add(6), 1)
                                    |
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                                 6 + 0

So, the final returned value is 12 + (7 – (13 – (4 + (11 – (6 + 0))))) = 15