Passage Summary: Scientists thought getting rid of viruses would help plankton grow, but it actually made the plankton population shrink.

Reasoning: Scientists removed all viruses from seawater expecting plankton to grow faster, but the plankton population actually decreased.

Analysis: We are looking for a way to resolve a paradox: why would removing a potential threat (viruses) harm the population? The solution must provide a hidden benefit that viruses offer to plankton. Perhaps the viruses kill off a different organism that eats plankton, or maybe the viruses are a necessary part of the plankton's nutrient cycle. Look for an answer that establishes a positive, indirect relationship between the presence of viruses and the survival of plankton.