Read these sentences from the text. "In the years right after the invasion, phytoplankton fell by 80 percent. Zooplankton (which eat phytoplankton) declined by half. And the smallest zooplankton (called microzooplankton), fell by about 90 percent." Based on these sentences, what does the word "decline" most nearly mean?
A. to drop in number
B. to fall over
C. to increase
D. to stay the same