The middle ear of the frog is externally closed by 

Options

A. mucus membrane
B. ciliated epithelium
C. tympanic membrane
D. nictitating membrane

Answer

To answer the question "The middle ear of the frog is externally closed by," we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Structure of the Ear**:
- The ear is typically divided into three parts: the outer ear, the middle ear, and the inner ear.

2. **Identify the Ear Structure in Frogs**:
- Frogs, being amphibians, have a slightly different ear structure compared to mammals. They possess only two parts: the middle ear and the inner ear.

3. **Focus on the Middle Ear**:
- The question specifically asks about the middle ear of the frog.

4. **Determine What Closes the Middle Ear**:
- In frogs, the middle ear is externally closed off by a specific structure.

5. **Identify the Structure**:
- The structure that closes the middle ear of the frog is known as the tympanic membrane.

6. **Conclusion**:
- Therefore, the middle ear of the frog is externally closed by the tympanic membrane.

### Final Answer:
The middle ear of the frog is externally closed by the tympanic membrane.

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