HSA 2006 Biology Item 13

Use the technical passage "The Good, The Bad, The Zebra Mussel" to answer the following:

A scientist was testing the hypothesis that zebra mussels cause a decrease in the population of bluegill, a type of fish. The mussels feed on copepods, a type of plankton that is also eaten by bluegill and other fish. In an experiment, he counted the number of bluegill and the number of copepods in an aquarium containing one kiloliter of water. He then added zebra mussels. After two weeks, and again after four weeks, he counted the copepods, bluegill, and zebra mussels. Which set of experimental data supports his hypothesis that the number of bluegill decline because zebra mussels eat copepods?