On a rainy day I was driving north through Vermont I noticed a young man holding up a si

问题补充：

On a rainy day I was driving north through Vermont, I noticed a young man holding up a sign “Boston”.

A. where; reading

B. when; reading

C. where; to read

D. when; to read

答案：

(答案→)A. where; reading

B. when; reading

C. where; to read

D. when; to read

解析：考查定语从句的同时考查非谓语动词的用法。where引导的非限定性定语从句修饰Vermont，现在分词reading表示主动，说明sign的内容。