Final answer:
B. Greater than atmospheric pressure
Expiration in the lungs occurs when intra-alveolar pressure is greater than atmospheric pressure, allowing air to flow out.
Step-by-step explanation:
Expiration occurs when the pressure inside the lungs, known as intra-alveolar pressure or intrapulmonary pressure, is greater than atmospheric pressure. This process enables air to flow out of the lungs. The atmospheric pressure is the pressure exerted by the surrounding air, and during expiration, it must be lower than the pressure inside the lungs for air to be expelled. Intrapleural pressure is always lower than intra-alveolar pressure, making it negative relative to both intra-alveolar and atmospheric pressures. Therefore, the correct answer is B. Greater than atmospheric pressure.