A good friend of mine from I was born showed up at my home right before I left for Beijing

问题补充：

A good friend of mine from I was born showed up at my home right before I left for Beijing．

A. how

B. whom

C. when

D. which

答案：

A. how

B. whom

C. when

D. which

解析：

考查定语从句的引导词。本句中的主语是A good friend of mine，谓语动词是showed up，中间from whenI was born为定语从句修饰friend，又因定语从句不缺成分，所以不用B和D，根据句意又排除A。