Although everything seems to have been taken into consideration, ______ accident can happen when

the astronauts walk in ______ space. [ ]

A. /; the

B. an; the

C. the; the

D. an; /

建筑立面图是一栋房屋的哪些面的投影图（）

A.侧立面和背立面投影图

B.正立面和侧立面投影图

C.立面和背立面投影图

D.正立面和东立面

E.侧立面和东立面