第17题: [单项选择]"Key" time is a period when ______. A. visitors come to the plants B. all workers must be at work C. employers go round in the workshops D. rush hours are over 参考答案:B 答案解析:从文章第二段第三句“These aye, firstly, that all workers must be present for certain "key" times in day”,我们可以猜