Mammals Have a Family Between 500 and 1000 Genes That

Mammals have a family between 500 and 1000 genes that encode receptor proteins on the surface of odor-receptor neurons. If all mammals have these genes, why do some mammals have a better sense of smell than others do?

A) Some of the genes actively transcribing receptor proteins in some species are completely absent in other species.
B) Some of the genes may have been mutated and rendered inactive in some species but not in others.
C) Some species, particularly humans, rely much less on odor detection for survival; thus, the genes have mutated to encode proteins that aid in other senses, such as sight.
D) Some species of mammals have a larger set of "basic odors" than other species.