How Can Variation in Human Skin Color Be Used as an Example

How can variation in human skin color be used as an example of both functional and genetic adaptations?

A) Mutations in skin cells caused by exposure to the sun produce increased genetic variation passed on to offspring.
B) Vasodilation and vasoconstriction of the blood vessels under the skin adapt to the amount of sunlight.
C) Human skin may be dark because of tanning or because of the genetically controlled production of more melanin.
D) Individuals exposed to plentiful sunlight may at first tan but eventually develop genes that code for more melanin.