17. My brother had become a doctor, ______ was what he wanted to be. A. who B. what C. that D. which

Which .定语从句有限制性和非限制性两种，限制性定语从句是先行词不可缺少的部分，去掉它主句意思往往不明确 ，限制性定语从句一般紧接在先行词的的后面。例如 The boys who were free would came along and made fun of him .有空的男孩子会来取笑他。
而非限制性的定语人名是先行词的附加说胆，去掉了也不会影响主句的意思，它与主句之前通常用逗号分开。
This novel ,which I have read three times ,is very touching 我本小说很动人，我已经读了三遍。
非限制性定语从句还能将整个句子做为先行词。Liquid water changes to vapor ,whic is called evaporation .液态水变为蒸汽，这就叫做蒸发。

which
非限定性定语从句起补充说明作用，缺少也不会影响全句的理解，在非限定性定语从句的前面往往有逗号隔开，如若将非限定性定语从句放在句子中间，起前后都需要用逗号隔开。
在非限定性定语从句中，关系词不能用that。

加了逗号了啊，一般加逗号隔开的都是非限制定语从句。