TOEFL Reading: ETS-TOEFL阅读机经 - U6F9W75QAF891U3CL$

According to paragraph 1, which of the following was true of small planktonic organisms at the end of the Cretaceous A. They lost their main sources of food. B. They developed calcareous shells. C. They decreased greatly in number as did many other types of organisms. D. They replaced other minute organisms as a food source in the major marine food chain.