— How many students are there in the classroom?

— _____ I don't know where they have gone. [ ]

A. No one.

B. None.

C. Nobody.

D. Nothing.

三（四）等水准测量时，读数时要先说明（），以便于记录。

A.黑面或红面

B.黑面

C.红面

D.正面