**Core Concept**
Helium is used in conjunction with oxygen in patients with fixed respiratory obstruction due to its lower density compared to air. This property reduces the resistance to gas flow, making breathing easier and more efficient.

**Why the Correct Answer is Right**
When a patient has a fixed respiratory obstruction, the airway resistance is significantly increased, leading to increased work of breathing. By using helium, which is less dense than air, the resistance to gas flow is decreased, allowing for easier breathing. This is particularly beneficial in patients with conditions such as bronchiolitis or severe asthma. The lower density of helium also reduces the pressure required to move air through the airways, making it a more efficient gas to use.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not mention the primary reason for using helium in patients with fixed respiratory obstruction, which is its lower density.
**Option B:** This option is incorrect because it does not accurately describe the benefits of using helium in this clinical scenario.
**Option C:** This option is incorrect because it is not directly related to the use of helium in patients with fixed respiratory obstruction.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that helium is often used in conjunction with oxygen in patients with severe respiratory distress, particularly those with conditions that cause fixed airway obstruction. This combination can significantly improve the patient's respiratory status.

**Correct Answer:** C.