If the operation is defined for all integers a and b by

If the operation is defined for all integers a and b by

If the operation is defined for all integers a and b by a@b = a + b – ab, which of the following statements must be true for all integers a, b and c? GMAT Explanation

If the operation is defined for all integers a and b by a@b = a + b – ab, which of the following statements must be true for all integers a, b and c?

I. a@b = b@a
II. a@0 = a
III. (a@b)@c = a@(b@c)

(A) I only
(B) II only
(C) I and II only
(D) I and III only
(E) I, II and III