维持胸内负压的必要条件是（）
A.胸膜腔密闭状态
B.呼吸肌收缩
C.胸廓扩张
D.呼吸肌舒张
E.呼吸道存在一定阻力
参考答案：A
—“Do you mind if I look at your notes?”
—“Of course not. _______.”
A．No problem
B．Be my guest
C．With pleasure
D．Not at all
