下列哪些因素可使肺顺应性降低的有（）

A.血气胸

B.肺不张

C.腹水或腹腔巨大肿瘤

D.CO₂气腹

E.俯卧位手术

In radiotelephone communications，the prefix PAN-PAN indicates that（）.

A.a ship is threatened by grave and imminent danger and requests immediate assistance

B.a calling station has an urgent message about the safety of a person

C.the message following the prefix will be about the safety of navigation

D.the message following is a meteorological warning