hat用法真有趣， 两个地方它不去;
逗号后边它不去， 介词之后不考虑;
That做为关系代词，不可用在非限制性定语从句中，即不可用在逗号后边;定语从句中介词之后也不可使用。在逗号或介词后，指物时用which,指人时用who或whom。
例句1)They arrived at a farmhouse, in front of ______sat a small boy.
A. whom B. who C. which D. that
解析：在介词of之后指物，选择C which。
例句 2)The engineer with _____my father works is about 50 years old.
A. whose B. who C. which D. whom
解析：选择 D whom。介词后指人。
例句 3)The letter is from my sister, ______is working in Beijing.
A. which B. that C. whom D. who
解析：选择 D who。逗号后指人，在定语从句中做主语。
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