(A) Source motion effects the wavelength but observer motion effects the number of waves received by him
(B) The apparent change in frequency depends on the distance between source and observer
(C) The echo and the direct note differ both in speed and intensity
The correct statements are

Options

A. Only A
B. B,C only
C. A,B, C
D. A,C only

Answer

To determine the correct statements among the given options, we will analyze each statement one by one.

### Step 1: Analyze Statement A
**Statement A:** "Source motion affects the wavelength but observer motion affects the number of waves received by him."

- When the source of sound is in motion, it causes a change in wavelength due to the Doppler effect. If the source moves towards the observer, the wavelength decreases, and if it moves away, the wavelength increases.
- When the observer is in motion, the rate at which they receive the wavefronts changes. If the observer moves towards the source, they encounter more wavefronts per unit time, effectively increasing the frequency perceived by the observer.

**Conclusion for A:** This statement is correct.

### Step 2: Analyze Statement B
**Statement B:** "The apparent change in frequency depends on the distance between source and observer."

- The apparent frequency (or observed frequency) does depend on the relative velocities of the source and observer, not the distance between them. The Doppler effect is primarily influenced by how fast the source and observer are moving relative to each other, rather than the distance separating them.

**Conclusion for B:** This statement is incorrect.

### Step 3: Analyze Statement C
**Statement C:** "The echo and the direct note differ both in speed and intensity."

- The speed of sound remains constant in a given medium regardless of whether it is a direct sound or an echo. Both the direct sound and the echo travel at the same speed.
- However, the intensity of the echo may be less than that of the direct sound due to absorption and scattering when it reflects off surfaces.

**Conclusion for C:** This statement is incorrect.

### Final Conclusion
The only correct statement is **A**.

### Summary of Correct Statements:
- **Correct Statement:** A
- **Incorrect Statements:** B, C

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