At the beginning of class, the noise of desks _______ could be heard outside the classroom.  [ ]

A. opened and closed

B. to be opened and closed   

C. being opened and closed

D. to open and close

轨束梁的容许跨度与轨束梁弯曲应力有关，支点中心距一般不宜超过（）m。

A.3.1

B.3.2

C.3.3

D.3.4