The apparent depth of water in cylindrical water tank of diameter `2R cm` is reducing at the rate of `x cm // min ` when water is being drained out at a constant rate. The amount of water drained in `c.c.` per minute is `(n_(1)=` refractive index of air, `n_(2)=` refractive index of water )

A. `xpiR^2n_1/n_2`

B. `xpiR^2n_2//n_1`

C. `2piR^2n_1//n_2`

D. `piR^2x`