Practice problem #14, page 340

This is a logic question really, but it's a pretty hard one by SAT standards.

You know that a(b - a) = 0, which means either a = 0, b - a = 0, or both. (You know this because when a product equals zero, that can only happen when at least one of the things being multiplied together is zero.)

Since we're given a > b, we know b - a can't be zero. So a has to be 0. The other two conditions follow from that:

I    a = 0 (we know this is true)
II   b < 0 (we know this is true because a = 0, and b is less than a)
III  a - b > 0 (we know this is true because 0 - a negative number will always be positive)