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。