B
[1]如果陈述句部分的主语是不定代词nothing,something,everthing时,后反意部分主语用it,如：
Nothing is too easy in the world,is it?
[2]如果陈述句部分主语是不定代词everyone,nobody,somebody,someone时,后反意部分主语用they（当强 调全体时）或he （当强调个体时）.如：
No one knows him,do they?
Someone is waiting for you,isn't he
[3]如果陈述部分主语是指示代词this,that,后用it；如果是指示代词these,those,则后用they.如：
This is a nice cap,isn't it?
These are boxes,aren't they?