This / That is a clock.
This / That is not / isn't a clock.
---Is this / that a clock? ---Yes it is. ---No, it isn't.
These/ those are clocks.
These/ those are not/aren't clocks.
---Are these/ those clocks? ---Yes they are. ---No, they are not.
注:在指示代词作主语时的疑问句的回答中 ,指示代词要换成人称代词。
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