Who is it？与 Who is that？的区别

有读者问：Who is it? 与 Who is that? 有什么不同？

Who is it? 中的 it 没有 Who is that? 中的 that 那样具体明确。it可指代任何人，如有人敲门，你即可问道： Who is it? that 则须指代具体“那个人”，如你在某一场合中见到一个人，但不知他或她是谁，便可问旁边的人说 Who is that?