elephants stripping bark from trees viruses infecting the cells of certain birds beavers constructing dams to make ponds for lodges semi-aquatic rodents feeding on the roots of aquatic plants
I gave my students this multiple choice question: Animals can affect their habitats through activities of finding food and raising their young. Sometimes these activities have a positive effect on the habitat of other organisms. Which is the best example of an activity that has a positive effect?

Only one answer is correct among these 4 choices:
- elephants stripping bark from trees
- viruses infecting the cells of certain birds
- beavers constructing dams to make ponds for lodges
- semi-aquatic rodents feeding on the roots of aquatic plants

Could you tell me which one is correct?

beavers constructing dams to make ponds for lodges