Longitudinal waves can propagate

Options

A. in a metal but not in a gas.
B. in a gas but not in a metal.
C. neither in a metal nor in a gas.
D. both in a metal and in a gas.

Explanation

### Step-by-Step Solution:

1. **Understanding Longitudinal Waves**:
Longitudinal waves are waves where the particle displacement is parallel to the direction of wave propagation. A common example of longitudinal waves is sound waves.

2. **Requirement of Medium**:
Longitudinal waves require a medium (solid, liquid, or gas) to propagate. This is because they rely on the interaction between particles in the medium to transfer energy.

3. **Propagation in Gases**:
In gases, particles are relatively far apart and can move freely. When a sound wave travels through a gas, it causes the gas particles to vibrate back and forth in the direction of the wave. This vibration allows the wave to propagate through the gas.

4. **Propagation in Metals**:
Metals have closely packed particles and free electrons. The particles in metals also vibrate back and forth, allowing longitudinal waves to propagate. The speed of sound in metals is generally higher than in gases due to the denser medium and the presence of free electrons, which facilitate quicker energy transfer.

5. **Conclusion**:
Longitudinal waves can propagate through both gases and metals. Therefore, the correct answer to the question is that longitudinal waves can propagate in both metal and gas.

### Final Answer:
Longitudinal waves can propagate in both metal and gas.

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