(1) The point (a,b) can be at quadrant II and IV, and this means that b is either positive or negative. INSUFFICIENT
(2) a<b doesn't tell us much. This can basically be true for all quadrants. INSUFFICIENT.
Together, you know that (a,b) is in quadrant II and IV. In IV, b is negative and a is positive. Since b>a, this means that it cannot be in quadrant IV. This leaves us with only quadrant II. In quadrant II, a is always negative, b is always positive.