托福 tpo 6 阅读!

问题描述:

托福 tpo 6 阅读!
certain fossils always appears in more ancient sediments,while others begin to be seen as the strata become more recent.
这句话后面是不是 有两个动词?begin 和 become 还是 有省略呢?

没问题,从句.
as不做成分引导的从句,相当于that