The "white chalk cliffs of Dover" are mentioned in line 14 of the passage to（） A: show where the plankton sediment first began to build up B: provide an example of a plankton buildup that scientists cannot explain C: provide all example of the buildup of plankton sediment D: indicate the largest single plankton buildup on Earth

海洋生物包括 A: Plankton B: Nekton C: Mammals D: Benthos