第31题: [单项选择]The entire morale among the employees in Factory B ______ until now ever since the introduction of our new flexible working hour system. A. were boosted B. had been boosted C. boosting D. has been boosted 参考答案:D 答案解析:空格内应填入谓语动词(这里需要单数形式的动词)。因为后面没有宾语,所以要考虑被动语态。最后,注意到和ever since的搭配,应选择现在完成时。另外,until now(直到现在)也是使用现在完成时