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IMO C ...

 

(-a,b) and (-b,a) would be in the same quadrant if ab > 0 i.e., either both a & b are positive or negative (for example, if a=1, b=2 then (-1,2) & (-2,1) are in the same quadrant)

 

(1) INSUFF because we can't say (-x,y) lies in the same quadrant just because x & y are of same sign ... we need to correlate it with a or b ...

(2) INSUFF because even if x & a are of the same sign, y & b can be same or different sign ...

 

Taking (1) & (2) it's clear that x & y are of the same sign as 'a' ... hence SUFF

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