What Causes Some People to Develop Absolute Pitch, the Ability

What causes some people to develop absolute pitch, the ability to name a tone (such as B#) after hearing it?

A) They have a rare gene that causes unusual development of part of the brain.
B) They have a rare gene that causes unusual development of the basilar membrane of the ear.
C) They overdevelop auditory sensitivity after damage to one of the other senses.
D) They practice the skill, usually beginning in early childhood.