—Where is Jack?I can’t find him anywhere.

—He__________ his homework upstairs.

A．might have done

B．must have done

C．must be doing

D．must do

风管系统安装后，必须进行严密性检验，合格后方能交付下道工序。风管系统严密性检验以（）为主要检验对象。

A.主、干管

B.完整风管系统

C.隐蔽的支管

D.风口连接处