新生儿呼吸窘迫综合征的主要病因是（）。
A.剖宫产和异常分娩
B.肺内液体过多
C.原发性肺不张
D.肺表面活性物质缺乏
参考答案：D
Never before ___________in greater need of technical experts than it is today.
A．has Shenyang been
B．Shenyang has been
C．was Shenyang
D．Shenyang was
