TOEFL Reading: ETS-TOEFL阅读机经 - _16YBQ44V7K4B3C5L$

According to paragraph 5, which of the following happened sometime between 1100 and 542 million years ago? A. A sudden explosion of life forms on land occurred together with a sharp decline of life in the water. B. Ultraviolet radiation became more harmful to living organisms. C. Molecular oxygen levels in the atmosphere stabilized, and ozone levels began to rise. D. Ozone reduced ultraviolet radiation to a level acceptable for life on land.