TOEFL Reading: ETS-TOEFL阅读机经 - 87D14U463691DH11L$

According to paragraph 4, banded-iron formations are found in what kind of rocks? A. Those that are more than 2.5 billion years old B. Those that do not contain oxidized compounds C. Those that are from a transitional period in terms of oxygen richness D. Those that are less than 1.8 billion years old