—May we leave classroom now?

—No, you        . You        to leave until the bell rings.

A．mustn’t, are allowed

B．don’t have to, are supposed

C．needn’t, aren’t allowed

D．can’t, aren’t supposed

（）灭火器灭火时导电，不可以扑灭电气设备火灾。

A.磷酸氨钠干粉

B.四氯化碳

C.二氧化碳

D.泡沫