GMAT Strategy

OK,

When do I draw the line on "yes or no" questions?

For examples,

If sum of the prime numbers x and y is an odd number, which of the following cannot be true?

I. y^2 is odd
2. x-y is odd
3. x/y is an integer

Now, I know that y^2 can or cannot be odd. Since the x and y has to be 2 and 3, 2x2 = even, 3x3 = odd. So it can be both.

So how do I interpret the question? since the original stem is asking which of the following cannot be true.. does that mean ALWAYS? POSSIBLY?

Thanks for your input guys.