The operation @ is defined by x @ y = 1/x + 1/y for all non zero x and y. If z is a number number greater than 1, which of the following must be true?
I. x @ (-z) = 0
II. z @ z/(z-1) = 1
III. 2/z @ 2/z = z
A. I only
B. I and II only
C. I and III only
D. II and III only
E. I, II, and III
I. x @ (-z) = 0
II. z @ z/(z-1) = 1
III. 2/z @ 2/z = z
A. I only
B. I and II only
C. I and III only
D. II and III only
E. I, II, and III
