— Who____ it be that is knocking at the door?

— It ____ be Father, but I'm not sure. [ ]

A. can; must

B. can; may

C. must; can

D. may; must

( )是指两个相邻的专业队进人流水作业的时间间隔，以符号“K”表示。

A．流水工期

B．流水参数

C．流水节拍

D．流水步距