Unlike inspiration, expiration is a passive act because no muscular contractions are involved. Expiration, however, depends on two factors. Which of the choices below lists those two factors?
combined amount of CO2 in the blood and air in the alveoli
the expansion of respiratory muscles that were contracted during inspiration and the lack of surface tension on the alveolar wall
the negative feedback of expansion fibers used during inspiration and the outward pull of surface tension due to surfactant
the recoil of elastic fibers that were stretched during inspiration and the inward pull of surface tension due to the film of alveolar fluid