The equation to the chord joining two points `(x_1,y_1)` and `(x_2,y_2)` on the rectangular hyperbola `xy=c^2` is: (A) `x/(x_1+x_2)+y/(y_1+y_2)=1` (B) `x/(x_1-x_2)+y/(y_1-y_2)=1` (C) `x/(y_1+y_2)+y/(x_1+x_2)=1` (D) `x/(y_1-y_2)+y/(x_1-x_2)=1`