30.Although all sedimentary rocks contain iron, but the deposits that are richest in iron consist predominatly of minerals such as iron oxides, carbonates, silicates, and sulfides.
答案是A 我开始选的C,现在明白C没错,OF藏在后面, 可是我还是不明白为什么是A?
31.In the early 1900's Pennsylvania's industries grew rapidly, a growth somethimes accompanied by disputes labor.