The author mentions "the green film of plant life that grows on stagnant pools" （lines 6－7

The author mentions "the green film of plant life that grows on stagnant pools" (lines 6-7) in order to explain ______.

A．how the sun affects lichens

B．why plants depend on water

C．where fungi become algae

D．what algae arc

请帮忙给出正确答案和分析，谢谢！