单选题 You ______ be tired, for you've only been working for an hour.
【正确答案】 C
【答案解析】本题主要考查情态动词，“情态动向+动词原形”才能作谓语动词，必须根据说话人语气，情态才能推断正确答案。根据you have only been working for an hour，由此可知选择表肯定的否定C。