For all numbers x and y, let the operation @ be defined by x @ y=xy-y. If a and b are positive integers, which of the following can be equal to zero?

I. a @ b

II. (a+b)@b

III. a@(a+b)

a) I only

b) II only

c) III only

d) I and II

e) I and III

Can you please explain why? Thanks alot!!