"Spring returns to the West Lake, and the spring water turns green" is the poet's description of spring. The "green" here refers to () A. algae B. ferns.

Algae mostly live in water, with simple structure, no differentiation in roots, stems and leaves, and chloroplasts in cells. In spring, when the temperature is right, algae in the water will multiply and float in rivers and lakes, making the water green. So the poem should describe seaweed. Ferns and bryophytes mainly live in wet places on land, and most seed plants live on land. It can be seen that a meets the meaning of the question.

So choose: C.