In quiet breathing (A) inspiration is passive and expiration involves muscular contractions. (B) inspiration and expiration involve muscular contractions. (C) inspiration and expiration are both passive processes. (D) inspiration involves muscular contractions and expiration is passive. (E) none of these.

In quiet breathing inspiration involves muscular contractions and expiration is passive.

Explanation:

Respiration process involves two different stages, inspiration - inhalation - and expiration - exhalation-.

During inspiration, the diaphragm contracts and goes down, while intercostal muscles also contract and go up. These actions enhance the size of the thoracic box and reduce internal pressure. As a result, air flows in and fills the lungs.

During expiration, the diaphragm relaxes and the volume of the thoracic box diminishes, which causes the pressure inside to increase. In consequence, the lungs contract and the air is expulsed outside.