GRE Reading Comprehension: PPO-GRE阅读PPO - _UQCMLK718CP_376R

The passage mentions that "Earth's atmosphere contains oxygen" primarily in order to A. contribute to a discussion of the nature of photosynthesis B. give an example of a chemical disequilibrium C. identify an element that scientists are looking for on Mars D. illustrate a process that occurs only on Earth E. introduce a contrast between oxygen and methane