21.下列那一種吐氣末正壓閥是一種閾式阻力（threshold resistor）閥且靠重力產生吐氣末正壓？
(A)Balloon type of expiratory valve
(B)Underwater column
(C)Spring-loaded diaphragm
(D)Restrictive orifice

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統計： A(13), B(50), C(19), D(6), E(0) #1057696