An industry has a water reservoir with a capacity of $900 \mathrm{~m}^{3}$. When there is a need to clean the reservoir, all the water must be drained. The water drainage is done by six drains, and takes 6 hours when the reservoir is full. This industry will build a new reservoir with a capacity of $500 \mathrm{~m}^{3}$, whose water drainage should be completed in 4 hours when the reservoir is full. The drains used in the new reservoir must be identical to those in the existing one. The number of drains in the new reservoir should be equal to (A) 2. (B) 4. (C) 5. (D) 8. (E) 9.
An industry has a water reservoir with a capacity of $900 \mathrm{~m}^{3}$. When there is a need to clean the reservoir, all the water must be drained. The water drainage is done by six drains, and takes 6 hours when the reservoir is full. This industry will build a new reservoir with a capacity of $500 \mathrm{~m}^{3}$, whose water drainage should be completed in 4 hours when the reservoir is full. The drains used in the new reservoir must be identical to those in the existing one.
The number of drains in the new reservoir should be equal to
(A) 2.
(B) 4.
(C) 5.
(D) 8.
(E) 9.