题目

When you’re invited to have dinner at home by an American friend, you should be ___ or a little later. It’s different from our Chinese custom. A. on time                 B. on business               C. on show 答案：A盖盖玻片时，必须让盖玻片的一边先接触液滴，然后再轻轻地盖上，这样做能（　　）A．避免盖玻片下面出现气泡B．防止实验材料移动C．增强透明度D．防止水溢出