In the Chalk Seas of Europe and the U

In the chalk seas of Europe and the U.S. Western Interior, the productivity of coccolithophores was greater during Late Cretaceous than any time in Earth's history, and this was due to the

A) general presence of oxygenated conditions on the seafloor.
B) very low ratio of magnesium to calcium in seawater during this time.
C) addition of fossil debris from bryozoans, arthropods, foraminifera, and others.
D) changing ratio of calcium carbonate to clay in the sediments.