The apparent depth of water in cylindrical water tank of diameter 2R cm is reducing at the rate of x cm/minute when water is being drained out at a constant rate. The amount of water drained in c.c. per minute is (n1 = refractive index of air, n2 = refractive index of water) [AIIMS 2005]

• A.            x p R2 n1/n2                           
• B.            x p R2 n2/n1
• C.            2 p R n1/n2                             
• D.            p R2 x