During quiet breathing, which of the following pressure gradients is positive?

The correct answer is B. Alveolar pressure during expiration.

Here's why:

• Atmospheric pressure is the baseline, considered zero for reference.
• Inspiration (breathing in): To draw air into the lungs, the alveolar pressure must become negative (lower than atmospheric pressure). The intrapleural pressure also becomes more negative during inspiration.
• Expiration (breathing out): To push air out of the lungs, the alveolar pressure must become positive (higher than atmospheric pressure). The intrapleural pressure remains negative throughout the breathing cycle in a healthy individual, though it becomes less negative during expiration.

Therefore, only alveolar pressure during expiration is positive compared to atmospheric pressure.