**Core Concept**
Heliox is a breathing gas composed of a mixture of helium and oxygen, used for its unique physical properties to improve respiratory function. **Helium** is an inert gas, which means it does not react with other elements under normal conditions. This property makes it safe for use in medical settings.

**Why the Correct Answer is Right**
Heliox is less viscous than air due to the lower molecular weight of **helium** compared to nitrogen, the primary component of air. This lower viscosity reduces airway resistance, making it easier for patients to breathe, especially in conditions like asthma or chronic obstructive pulmonary disease (COPD). The correct answer includes the fact that **helium** is inert and that heliox reduces the work of breathing.

**Why Each Wrong Option is Incorrect**
**Option C:** is incorrect because **heliox** actually has a lower density than air, not higher, due to the lower molecular weight of **helium**.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that heliox, due to its lower viscosity and density, can significantly reduce the work of breathing in obstructive lung diseases, but it does not increase oxygenation on its own since it is a mixture of **helium** and **oxygen**.

**Correct Answer:** D. b, d, e