Some traits, such as earlobe attachment, blood type, and the presence or absence of dimples, have limited variation throughout the human population because they are controlled by a single gene. Other traits, such as skin color and human height, are widely variable throughout the human population because.

A. they can not be affected by environmental factors

B. They are controlled by only one gene

C. They are controlled by more than one gene

D. They are not controlled by genes in any way