Tom had been studying French hard for nearly three years he mastered it．
A. after
B. when
C. since
D. before
网友回答
A. after
B. when
C. since
(答案→)D. before
解析：
试题分析:句意：——汤姆努力学了将近三年后才精通了法语。A在……之后；B当……时；C自从；D在……之前。根据前后句时态可知“学习法语”发生在“掌握法语”之前，所以用before，答案选D。