A negative impact of a large, hydroelectric dam is that it backs up water sometimes for miles behind it, destroying natural habitat that once was there.

But a positive environmental impact of a dam is that
A.
it makes it easier for migrating fish to make it upstream to reproduce

B.
it produces a lake, which is great for water skiing

C.
large amounts of electricity can be produced, without contributing to air pollution

D.
beavers won't build dams of their own