!help me fill in the blank!

Which of the following is a possible reason for sleep? a) growth b)digestive c)speed d)intelligence

What is hydroelectric power produced by?

Why do leaves change color in the fall? green pigments break down and no longer mask the color of chlorophyll. chlorophyll breaks down and no longer masks the colors of other pigments. red- and yellow-colored pigments grow and mask green-colored chlorophyll. green-colored chlorophyll breaks down and turns red and yellow.