This text explains a cause-and-effect pattern in the Hudson River ecosystem that began with the zebra mussel invasion. What effect did the zebra mussels have on the phytoplankton in the Hudson River?

A. The number of phytoplankton in the river rose by a little.
B. The number of phytoplankton in the river fell by a little.
C. The number of phytoplankton in the river rose by a lot.
D. The number of phytoplankton in the river fell by a lot.