If the $(x,y) $ is equidistant from the points $(a+b,b-a)$ and $(a-b,a+b)$ then $bx=$________

Question

$\begin{array}{1 1}(A) \; \large\frac{a}{y}\\(B)\;\large\frac{-y}{a}\\(C)\;ay\\(D)\;-ay \end{array}$