Which of the following statement(s) is/are true about sound waves?
(I) They are produced by vibrating objects.
(II) They can travel through a vacuum.
(III) They are longitudinal waves.
(IV) The speed of sound is highest in gas and lowest in a solid.

Options

A. I and II
B. I and III
C. I and IV
D. III and IV

Explanation

To determine which statements about sound waves are true, we will evaluate each statement one by one.

### Step 1: Evaluate Statement I
**Statement I**: "They are produced by vibrating objects."
- Sound waves are indeed produced when objects vibrate. When an object vibrates, it causes the particles of the medium (air, water, or solid) around it to also vibrate, creating sound waves.
- **Conclusion**: This statement is **true**.

### Step 2: Evaluate Statement II
**Statement II**: "They can travel through a vacuum."
- Sound waves are mechanical waves that require a medium (solid, liquid, or gas) to propagate. They cannot travel through a vacuum because there are no particles to transmit the sound.
- **Conclusion**: This statement is **false**.

### Step 3: Evaluate Statement III
**Statement III**: "They are longitudinal waves."
- Sound waves are classified as longitudinal waves because the displacement of the medium's particles is in the same direction as the wave propagation. This means that the compressions and rarefactions of the wave move in the same direction as the sound travels.
- **Conclusion**: This statement is **true**.

### Step 4: Evaluate Statement IV
**Statement IV**: "The speed of sound is highest in gas and lowest in a solid."
- This statement is incorrect. The speed of sound is actually highest in solids and lowest in gases. This is due to the density and elasticity of the medium; solids have tightly packed particles that transmit sound faster than the more loosely packed particles in gases.
- **Conclusion**: This statement is **false**.

### Final Summary of Statements
- **Statement I**: True
- **Statement II**: False
- **Statement III**: True
- **Statement IV**: False

### Final Answer
The true statements about sound waves are **I and III**.