TOEFL Reading: ETS-TOEFL阅读机经 - 3S1X1A7IP55ME8S7L$

According to paragraph 4, what can be learned from the type of iron compounds in seafloor rocks? A. How the process of photosynthesis has changed over time B. The level of oxygen in the water at a certain time in history C. How levels of iron in ocean water decreased over time D. The overall mineral content of the ocean water