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
网友回答
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。