An average human adult radiates about 100 W energy mainly in infra-red region of the electromagnetic spectrum. 50 persons are sitting in a hall with an air conditioning system which is 50 percent efficient. How much electricity must be used to maintain temperature of the hall at $$25^\circ\mathrm{C}$$ for 4 hours?

A

5 units

B

10 units

C

20 units

D

40 units