Dams are used to produce hydroelectric power. Which of these best describes the impact of a dam on the environment?

a. It causes air pollution.

b. It reduces the level of groundwater.

c. It causes changes in the river ecosystem.

d. It reduces the level of oxygen in river water.

The answer is c. It causes changes in river ecosystem

The answer is (C) because a dam doesn't have anything to do with air so eliminate (A). Reduce means decrease or lower but a dam is groundwater so eliminate (B). It is changing the river ecosystem because otters build them and they aren't naturally made so (C) is correct. And you can also eliminate (D) because the oxygen level doesn't lower simply because otters move and block off water ways to make their home.