Question

algae do not live very long, which sounds like a good thing for oceans. they fall to the bottom of the ocean and decay, or break down, when they die. however, during the decaying process, the dead algae use more oxygen! the decay of alg is another factor that lowers the levels of oxygen in the ocean. this can cause ocean life to die or, if they are able, to swim to other areas. how do dead algae make dead zones worse? select one: algae have a long life. the decaying algae use oxygen. fish eat decaying algae. dead algae keep water from circulating.

Explanation:

The text clearly states that during the decaying process, dead algae use more oxygen, which lowers ocean oxygen levels. This is the key factor that makes dead zones worse. The other options are either contrary to the text (Algae have a long life - text says algae do not live very long) or not mentioned (Fish eat decaying algae, Dead algae keep water from circulating).

Answer:

The decaying algae use oxygen.