问题
选择题
Tom had been studying French hard for nearly three years he mastered it．
A．after
B．when
C．since
D．before
答案
答案：D
题目分析: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。