Why is it important to use the active voice?

Writing in the active voice means constructing sentences where the subject “acts”:

In each of these sentences, the subject (I, You and Ben respectively) performs the action of the verb (threw, making, will watch). The sentences are punchy, direct and make it clear who’s doing what.

Writing in the passive voice means constructing sentences where the subject is “passive” – acted upon, rather than agents of action. For many forms of writing, this can create an undesired effect: sentences often become confusing or simply dull.

In each of these sentences, the subject (“the ball”, “too much noise”, “popcorn” and “a movie”) is being acted upon by the verb. With sentences written in this way, we can even eliminate the agent who is performing this action:

These are all perfectly correct sentences, but the reader has the sense that something is missing. Who threw the ball? Who or what is making too much noise? And surely someone’s going to watch that movie and eat that popcorn?

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."