If both x and y are positive even integers, then which of the following ... ...

If both x and y are positive even integers, then which of the following expressions must also be even?
I. yx-1              II.  y – 1            III.  x/2
(A) I only
(B) ...
(C) ...
(D) ...
(E) ...

*This question is included in Nova Press: Set B - Substitution