## Core Concept
Normal expiration, also known as quiet expiration, is a passive process that occurs when the diaphragm and other inspiratory muscles relax. This process is primarily driven by the elastic recoil of the lungs and the chest wall.

## Why the Correct Answer is Right
The correct answer, , states that expiration is a passive process. During normal expiration, the diaphragm and intercostal muscles relax. The elastic lungs recoil inward, and the chest wall recoils outward but then inward, decreasing the thoracic volume. This decrease in volume increases the pressure inside the lungs compared to the outside, causing air to flow out of the lungs.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it suggests that expiration is an active process, which is not true for normal, quiet expiration. Active expiration involves the contraction of muscles such as the abdominal muscles and internal intercostal muscles, typically occurring during forced expiration.
- **Option B:** This option is incorrect because it implies that expiration involves the contraction of specific muscles for its mechanism, which misrepresents the passive nature of normal expiration.
- **Option C:** This option might seem plausible but does not accurately describe the primary characteristic of normal expiration being a passive process driven by elastic recoil.

## Clinical Pearl / High-Yield Fact
A key point to remember is that during **normal, quiet expiration**, no muscles are actively contracted; the process relies on the elastic properties of the lung and chest wall. This contrasts with **forced expiration**, where muscles like the internal intercostals and abdominal muscles are engaged.

## Correct Answer: .
Passive process