Human populations in low-UV environments tend to have more lightly pigmented skin. One explanation is that the selective pressure for dark skin decreases as UV intensity decreases. At the same time there is selection for lighter skin to absorb more UV radiation, which is needed for vitamin D production. What evidence supports the vitamin D hypothesis for the evolution of lighter skin tones? a. lightly pigmented people tend to be at higher risk of vitamin D deficiency b. red hair and light skin are typical of people of northern European descent c. lightly pigmented people cannot synthesize vitamin D in high-UV environments, such as equatorial Africa d. indigenous peoples living at high latitudes have darker skin, but they eat foods rich in vitamin D