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.

The best explanation for this is that Indigenous peoples living at high latitudes have darker skin, but still they eat foods rich in vitamin D

Explanation:

Based on the question, two factors are responsible for evolution of light skin.

(a) The  selective pressure of low light intensity which acted against the dark skin pigment , in direction  of  the light skin. so that there is a directional  selection for light skinned pigment evolution.

( b)The stimulation of Vitamin  D production due to low UV exposure. Thus interaction between low UV and Vit, D are selective pressures for light skin.

Therefore  individuals who  are exposed to high light intensity, will have low   Vit,D production , and will be dark( since vit D synthesis is only at low light intensity). Thus it is the exposure to low -UV that selected for light-skin, which   leads to Vit.D production, and evolution of light skin.

Therefore indigenous peoples living at high latitudes have darker skin,(from high-UV exposure, and therefore low UV; to compensate for this  they eat foods rich in vitamin D.

This demonstrated; that it  is the interaction of low-UV (as selective pressure)and   Vit.D synthesis in the skin ,that facilitate  directional selection for light skin.

Explanation:

i'd think it's paralell

white blood cells or wbc s are mainly 2 types-

1.agranulocytes

2.granulocytes

the monocytes or macrophages of agranulocytes , dendritic cells which are accessory leucocytes, and b cells which is one type of lymphocytes   identify the antigens present on the pathogen's surface. these three cells are called antigen presenting cells(apc) which are phagocytes and destroy the antigens by secreting antitoxins. besides this leukocyte is always plays vital role in the immunity of the body.