After exchange of gases, when floor of buccopharyngeal cavity is lowered, air passes from 

Options

A. buccopharyngeal cavity to lungs
B. buccopharyngeal cavity to nostrils
C. external nostrils to outside
D. all the above

Answer

**Step-by-Step Solution:**

1. **Understanding Frog Respiration:**
- Frogs have a unique respiratory system that allows them to breathe both in water and on land. They can respire through their skin when submerged and use lungs when on land.

2. **Buccopharyngeal Cavity:**
- The buccopharyngeal cavity is the space in the frog's mouth that plays a crucial role in respiration.

3. **Mechanism of Air Intake:**
- When the frog wants to take in air, it lowers the floor of the buccopharyngeal cavity. This action expands the throat and creates a vacuum that allows air to enter through the nostrils.

4. **Air Flow Direction:**
- After the air enters the buccopharyngeal cavity, the nostrils close, and the floor of the mouth contracts, forcing the air into the lungs.

5. **Conclusion:**
- Therefore, when the floor of the buccopharyngeal cavity is lowered, air passes from the buccopharyngeal cavity to the lungs.

**Final Answer:**
- Air passes from the buccopharyngeal cavity to the lungs.

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