In negative pressure breathing, inhalation results from :

Options

A. forcing air from the throat down into the lungs
B. contracting the diaphragm
C. relaxing the muscles of the rib cage
D. contracting the abdominal muscles

Explanation

### Step-by-Step Solution

1. **Understanding Negative Pressure Breathing**:
- Negative pressure breathing is a mechanism used by many organisms, including humans, to inhale air. It relies on creating a pressure difference between the atmosphere and the lungs.

2. **Role of the Respiratory System**:
- The primary function of the respiratory system is to facilitate gas exchange, which involves taking in oxygen and expelling carbon dioxide.

3. **Anatomy of the Lungs**:
- The lungs are located in the thoracic cavity and contain alveoli, which are the sites for gas exchange. The lungs are surrounded by an airtight thoracic chamber.

4. **Mechanism of Inhalation**:
- During inhalation, the diaphragm, a dome-shaped muscle located beneath the lungs, contracts. This contraction increases the volume of the thoracic cavity.

5. **Effect of Volume Change**:
- As the volume of the thoracic cavity increases, the volume of the lungs also increases. This increase in lung volume leads to a decrease in pulmonary pressure (the pressure inside the lungs).

6. **Pressure Gradient Creation**:
- The decrease in pulmonary pressure creates a negative pressure relative to the atmospheric pressure outside the body. This pressure gradient causes air to rush into the lungs.

7. **Conclusion**:
- Therefore, in negative pressure breathing, inhalation results from the contraction of the diaphragm, which increases the thoracic volume and decreases lung pressure, allowing air to flow in.

### Final Answer
In negative pressure breathing, inhalation results from the contraction of the diaphragm.

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