[求助]想不通的几道题。。。年代未知

1.Today's nuclear fission fuels are the remnants of which used to be a much more active mixture of radioactive and fissionable materials two billion years ago and earlier.

为什么选A？

2.Petrography concerns primarily with the detailed description and classification of rocks , whereas petrology deals primarily with rock formation.

为什么选A？

3.Nebraska has floods in some years,_______.

A>in others drought             B>others in drought         

为什么选A？

我同意楼上的

2.Petrography concerns primarily with the detailed description and classification of rocks , whereas petrology deals primarily with rock formation.

为什么选A？

第三题的结构能分析一下吗？

为什么可以直接用in others drought?

drought不是名词吗？

flood不是名词吗？省略相同谓语了。