我告诉你吧.绝对不能用this 和that来回答.为了避免重复,应该把this和that改为it.即：It is a/an +名词（单数）.It is 可以缩略成It’s.
如：What is this?It’s a book.
又如：What is that?It’s a pen.
跟它类似的还有What are these?What are those?答语都用They're 不能用These are 和 Those are.