Let `A B C` be a triangle. Let `A` be the point `(1,2),y=x` be the perpendicular bisector of `A B ,` and `x-2y+1=0` be the angle bisector of `/_C` . If the equation of `B C` is given by `a x+b y-5=0` , then the value of `a+b` is (a)`1 `(b) `2 `(c) ` 3` (d) `4`