The wreckage of something like a jet plain discovered in the heart of the desert is believed ______ a UFO and _______down for reasons still mysterious.     [ ]

A. to be; have fallen  

B. to have been; to have fallen  

C. to be; to fall  

D. having been; having fallen

胸部CT扫描时，病人最佳呼吸状态应是（）

A.深呼气后屏气扫描

B.深吸气后屏气扫描

C.捏捂病人鼻口扫描

D.平静口式呼吸扫描

E.采用腹式呼吸扫描