问题描述：
who is that和who is it
假如：A：There is someone knocking at the door.Who is that/it?用哪个?
B：That/it may be John用哪个?
假如：A：There is someone knocking at the door.Who is that/it?用哪个?
B：That/it may be John用哪个?
问题解答：
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