Three years had gone by he came to realize what a fool he was not to listen to the teache

问题补充：

Three years had gone by he came to realize what a fool he was not to listen to the teachers !

A. until

B. since

C. before

D. after

答案：

A. until

B. since

(答案→)C. before

D. after

解析：考查连词，until 直到，since 自从，before 在什么之前， after在什么之后，句意：自从他意识到多么的愚蠢呀他没有听老师的话三年已经过去了。所以选C。