单选题 Two months ago, we were very bearish, but ______ then we have switched our strategy to the bullish side.
A. yet
B. still
C. already
D. since

【正确答案】 D

【答案解析】高级问题这里的then是名词，它的前面应该填入介词。选项中只有D选项是介词，其他选项都是副词。
[高频词语] since then 从那以后 not yet 还没有(做某事) is still+ -ing 仍在做某事 have already +p.p. 已经完成了某事 bearish (股票)看跌的 bullish (股票)看涨的