A frog must swallow air to expand the lungs because frog

Options

A. Has no diaphragm
B. Has no vagus nerve
C. Is relatively primitive vertebrate
D. Normally breathe though its skin

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks why a frog must swallow air to expand its lungs. This implies that there is a specific anatomical or physiological reason related to the frog's respiratory system.

2. **Identifying the Key Feature**: Frogs do not have a diaphragm. The diaphragm is a muscle that helps in the breathing process by contracting and relaxing to change the volume of the thoracic cavity, which in turn helps expand and contract the lungs.

3. **Role of the Diaphragm**: In organisms that possess a diaphragm (like humans), this muscle aids in drawing air into the lungs and pushing it out by changing the pressure within the thoracic cavity. Since frogs lack this structure, they cannot use this method to ventilate their lungs.

4. **Mechanism of Breathing in Frogs**: Frogs primarily use their lungs for respiration, but without a diaphragm, they cannot actively expand their lungs. Instead, they must swallow air to fill their lungs. This action allows them to increase the volume of air in their lungs, which is necessary for respiration.

5. **Additional Functions of Lungs**: The lungs in frogs also play a role in buoyancy while swimming. When the lungs are filled with air, they help the frog stay afloat in water.

6. **Conclusion**: Therefore, the correct answer to the question is that a frog must swallow air to expand its lungs because it has no diaphragm.

### Final Answer:
A frog must swallow air to expand the lungs because it has no diaphragm.