---Mum, I’m going out with my classmate tonight.

---OK. But you’d better _____ back too late.

A．don’t come

B．not to come

C．not coming

D．not come

关于空气净化技术，叙述正确的是

A．空气处于层流状态，室内容易积尘

B．洁净室不能按洁净度等级的高低依次相连

C．层流净化技术只能达到10000级

D．乱流可获得100级的洁净空气

E．洁净室必须保持正压