How Is Expression of OR Proteins Controlled

How is expression of OR proteins controlled?

A) Expression is controlled by stimuli from the environment.
B) Expression is activated by feedback mechanisms from the olfactory sensory system.
C) Each olfactory neuron produces only one OR receptor protein.
D) All neurons produce the OR proteins, as there is no reason to increase or decrease expression.
E) Each neuron produces a subset of related OR proteins, under control of a common stimulus.