To Find the Number of Electrons Per Unit Volume with Energy

To find the number of electrons per unit volume with energy between E and E + dE in a metal we must multiply the number of allowed states per unit volume with energy E by

A) the probability that a state is unoccupied, .
B) the probability that a state is occupied, .
C) the probability that a state is unoccupied, .
D) the probability that a state is occupied, .
E) dE alone.