Dogs Are More Sensitive at Detecting Odors Than Humans Because

Dogs are more sensitive at detecting odors than humans because:

A) dogs have more types of receptors but the same overall number of receptors.
B) both have the same number of receptors. However, more sophisticated processing occurs at the cortical level in dogs.
C) dogs have fewer types of receptors, so they become less confused between smells.
D) dogs have as many as 1 billion receptors, while humans have only 3-7 million.