问题补充：
This is a very convenient place to live in,is an airport connecting all the big cities.
A. 20 km northwest of it
B. 20 km northwest of which
C. northwest 20 km to it
D. northwest 20 km of which
答案：
A. 20 km northwest of it
(答案→)B. 20 km northwest of which
C. northwest 20 km to it
D. northwest 20 km of which
解析：句意：这是一个非常方便生活的地方，西北的20千米处，有一个机场连接了所有的大城市。通过分析句子结构，可知这个句子中包含了一个非限制性定语从句，“西北的20千米处”，应该说20 km northwest of which，选B。
