34. My friend Peter, ______ had been on a visit to America, returned yesterday.
A. which
B. that
C. who
D. whose

解析：
该题考查的是定语从句的关系代词。
该句的先行词是Peter，指人的关系代词有who、whose、whom、that。
在非限制性定语从句中不能用that，且先行词在从句中作的是主语，所以选C (who)。
参考译文：
我那去美国旅行的朋友，皮特，昨天回来了。
拓展：
指人时，用who不用that的其他情况：
1．先行词为one、ones或anyone
Anyone who breaks the law should be punished.
2．先行词为these时
These who are going to Beijing are the best students of our school.
3. 在there be 开头的句子中
There is a student who wants to see you.
4．一个句子中带有两个定语从句，其中一个定语从句的关系代词是that，另一个宜用who，以免重复。