That passenger was very impolite to the conductor of course made things even worse.A.

问题补充：

That passenger was very impolite to the conductor, , of course, made things even worse.

A. who

B. which

C. what

D. that

答案：

A. who

(答案→)B. which

C. what

D. that

解析：考查非限制性定语从句，which引导非限制性定语从句，可以修饰前面的句子,句意：这个乘客对乘务员不礼貌，这使得事情更糟。选B。