土壤氧化还原体系
参考答案：
土壤中可同时存在氧化态和还原态的某些物质系列。如土壤中的铁体系、锰体系，硫体系、氮体系、碳体系等。
简答大事记的界限。
—How do you usually study for a test?
—I study_______. [ ]
A. with seeing English-language videos
B. with watching English-language videoes
C. by watching English-language videoes
D. by watching English-language videos
