Q: Which process in breathing is considered passive?  

Answer: B

During normal expiration, the muscles involved in breathing, including the diaphragm and external intercostal muscles, relax without any active contraction. This relaxation results in the decrease of the thoracic cavity volume and the consequent expulsion of air from the lungs. As no muscle contraction occurs during normal expiration, it is considered a passive process. On the other hand, inspiration, forceful inspiration, and forceful expiration all require active muscle contractions, which require the expenditure of energy.

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