Ofthesetwoadvantages,Iprefertheformer.是个什么句型,Of在本句中起什么作用?

Question

Of these two advantages,I prefer the former.是个什么句型,Of在本句中起什么作用?

Answer 1

这个句子前面肯定有提到specific two advantages,所以才紧接着衔接了of these two advantages,表达在两个优势中.当然也可以说of which...表达同样的意思.
举例：there are 30 students in our class,of which 14 ...boys and 16.