The plates each of area `A` of a parallel plate caoacitor are given charges `Q` and `-Q`. The plates are joined by a non-conducting spring of force constant `k`. The natural length of the spring is `d`, the inital separation between the plates. The left plate is connected to a vertical wall through a massless non-conducting rope and the right plate is connected to a block of mass `m` through similar rope. Assume the pulley to be massless, neglect dielectric effect of the spring and `Q^(2) lt 2mAepsilon_(0)g`
If `Q^(2)=2mAepsilon_(0)g`, the elongation of the string is

Options

A. zero
B. `(2)/(k)(mg-(Q^(2))/(Aepsilon_(0)))`
C. `(2)/(k)(2mg-(Q^(2))/(Aepsilon_(0)))`
D. None of these

Explanation

Use force balance.