May I have you attention please? I have few things to tell you. When you have class here in the language lab, please be here a little early. Change your shoes before enter the lab. Don’t bring anything here except your text books. When you are into, please don’t touch these machine without permission. Always do as the teacher tell you to. In class you should only speak the English, not Chinese. Recorders can be used make a copy of the listening materials. When class over, turn off your machines and leave the lab one by one.

That’s all. Thank you.

对革兰阳性菌作用强、革兰阴性杆菌作用弱的是（）

A．第一代头孢菌素

B．第二代头孢菌素

C．第三代头孢菌素

D．第四代头孢菌素

E．单环β-内酰胺类