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We know that the scientists expected the growth rate of the plankton population in the water to increase dramatically when they removed all viruses from a seawater sample. But, the population actually got smaller. To help explain the unexpected result, the correct answer must touch upon some way that the viruses in the seawater helped the plankton. This is why answer choice (C) is correct. If the plankton "utilize nutrients released by the death of organisms killed by viruses," then the complete eradication of the viruses will leave the plankton in need of nutrients. Therefore, answer choice (C) is correct because it most helps to explain the unexpected result in the experiment. Hope that helped! Please let us know if you have any other questions.

I chose D as answer but wanted further explanation on why answer choice A couldn't work?

@ulino23, Let's look at (D) and (A). (D) says, "The absence of viruses can facilitate the flourishing of bacteria that sometimes damage other organisms." (D) is a tempting answer choice, but the problem is that we don't know if these other organisms are killing the plankton at a higher or lower rate than the viruses were killing them at. Additionally, we don't know if plankton are among the types of organisms that are damaged by these bacteria, so we can get rid of this answer choice. (A) says, "Viruses in seawater help to keep the plankton population below the maximum level that the resources in the water will support." (A) makes the paradox stronger by providing a good reason to think that the plankton would thrive because it's telling us that there will be enough food for the plankton that do survive since the viruses will keep their population below the level that would interfere with their ability to get the resources they need to live. Thus, (A) is out. Hope this helps. Let us know if you have any other questions!