A storage tank is a right circular frustum 30 cm tall with a bottom radius of 10 cm and a top radius of 20 cm. Water flows into the tank at 1000 cm³/min. What is the rate of rise of the water level when the depth of the water is 15 cm?
a) 9π4 cm/min
b) 9π40 cm/min
c) 49π160 cm/min
d) 27π40 cm/min