One Reason Why We Have Difficulty Tasting Low Concentrations of a Bitter

One reason why we have difficulty tasting low concentrations of a bitter chemical is that:

A) we have dozens of different types of bitter receptors.
B) bitter receptors are located only on the back of the tongue.
C) bitter receptors have axons with very slow conduction velocities.
D) each bitter receptor responds to a wide variety of chemicals.