Earthquake produces which kind of sound before the main shock wave begins :

Options

A. ultrasound
B. infrasound
C. audible sound
D. None of these

Explanation

To solve the question about the type of sound produced by an earthquake before the main shock wave begins, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks what kind of sound is produced by an earthquake prior to the main shock wave. We need to analyze the options given.

2. **Analyzing the Options**:
- **A. Ultrasound**: This refers to sound waves with frequencies above the audible range (above 20 kHz). Earthquakes do not produce ultrasound.
- **B. Infrasound**: This refers to sound waves with frequencies below the audible range (below 20 Hz). Earthquakes are known to produce infrasound, especially at low frequencies.
- **C. Audible Sound**: This is the range of sound that can be heard by the human ear (20 Hz to 20 kHz). While earthquakes can produce audible sounds, the question specifically asks about the sound before the main shock.
- **D. None of These**: This option suggests that none of the previous options are correct.

3. **Identifying the Correct Answer**: Based on the analysis, the sound produced by earthquakes before the main shock is primarily infrasound, which is characterized by low frequencies (around 5 Hz).

4. **Conclusion**: Therefore, the correct answer to the question is **B. Infrasound**.

### Final Answer:
The sound produced by an earthquake before the main shock wave begins is **infrasound**.

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