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

The right option is; C. They are controlled by more than one gene  

Traits such as skin color, human height and eye colour are known as polygenic traits. These traits have wide variation throughout the human population and are controlled by two or more genes. For example the human height is controlled by at least three genes with six alleles. The quantities of the genes are usually large but often have a little effect.

when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.