A（敲门）

B：Who is that？

A:It's Eva.(或I'am Eva)

2、打电话的时候

A（打电话）

B：（接电话）Who is that?

A:This is Eva

3、日常交谈,问“他/她/那是谁?”

A：Who is that?

B :He/She is Eva.(或That is my sister)

也有这么说的：

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。
　　

请问到底哪一种呢？谢谢。