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

在加强子宫收缩的方法中，下列哪项应专人监护（）。

A.灌肠

B.人工破膜

C.导尿

D.缩宫素静脉滴注

E.去卫生间