How are traces of ancient phytoplankton used to determine cooler and warmer water temperatures?

A. Chemical traces of phytoplankton that remain after millions of years are analyzed for data indicating water temperatures.
B. Climatologists analyze how much photosynthesis occurred after millions of years as an indicator of warmer water.
C. Large numbers of phytoplankton in fossil records after millions of years indicate warmer waters.
D. Fossil records indicate different species of phytoplankton prefer warm water to those that prefer colder water.