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^(2)n_(1))/n_(2)`
B. `(xpiR^(2)n_(2))/n_(1)`
C. `(2piRn_(1))/n_(2)`
D. `piR^(2)x`