5．当肋间外肌和膈肌收缩时，外界、气管、肺泡内的气压关系是

A．外界＞肺泡＞气管                                   B．气管＞肺泡＞外界

C．外界＞气管＞肺泡                                    D．肺泡＞气管＞外界