Solve this:

Q. The work don required to increase the separation distance from x₁ to x₁ + d between two masses m₁ and m₂ is 

     (a) – G m₁ m₂ [d³ – 3x₁ d (x₁ + d)] / [d(x₁ + d)]³

     (b) – G m₁ m₂ [$x_1^2$ – 2x₁ d (x₁ + d)] / [x₁(x₁ + d)]³

     (c) +G m₁ m₂ [$x_1^3$ + 3x₁ d (x₁ + d)] / [d(x₁ + d)]³

     (d) + G m₁ m₂ [d³ + 3x₁ d (x₁ + d)] / [x₁  (x₁ + d)]³