Skin color in humans is represented in geography by a cline, or clinal pattern. The skin color cline means that:
A. A certain skin color may be more common in one region than another, but the variation is gradual and continuous.
B. All of the people living in one region will share the same skin color, and people living in another region will share a different skin color.
C. People living at high altitudes will have a lighter skin color than those who live at sea level.
D. No patterns in skin color are visible in any regions of the world.