Water is poured into an inverted conical vessel of which the radius of the base is 2m and height 4m, at the rate of 77 litre/minute. The rate at which the water level is rising at the instant when the depth is 70 cm is (use d=22/7 )
(A) 10 cm/min
(B) 20 cm/min
(C) 40 cm/min
(D) 30 cm/min