The image of `P(a ,b)` on the line `y=-x` is `Q` and the image of `Q` on the line `y=x` is `R` . Then the midpoint of `P R` is (a) `(a+b ,b+a)` (b) `((a+b)/2,(b+2)/2)` (c) `(a-b ,b-a)` (d) `(0,0)`