The apparent depth of water in cylindrical water tank of diameter $2R\text{cm}$ is reducing at the rate of $x\text{cm/minute}$ when water is being drained out at a constant rate. The amount of water drained in $\text{c.c.}$ per minute is ($n_1=\text{refractive index of air}$, $n_2=\text{refractive index of water}$):

1. A
   $\frac{x\pi R^2{n}_1}{n_2}$
2. B
   $\frac{x\pi R^2{n}_2}{n_1}$
3. C
   $\frac{2\pi R{n}_1}{n_2}$
4. D
   $\pi R^{2}x$