I think the difference being must be true and could be true.

This is my thought:

If the question said could be true then I and III might be correct but since its says must be true (should apply to all cases)

For example if r = .9 s =1.9 then r-s = 1 which is equal to 1 and not less than 1

r/s is still less than 1

If u get down to 2 answer choices for must be true question pick couple of other numbers to see if they are still good(good to pick numbers from the extremes .1 and 1.9, .9 and 1.9 etc...)

Hope this helps!

I got it now, thanks.