The sound of an electricity bell dies down slowly when air is slowly pumped out from a bottle because

Analyze the question: An electric bell rings inside a bottle (a bell jar experiment). As air is pumped out, the sound fades gradually. We need the reason.

Concept: Sound is a mechanical (longitudinal) wave. It travels by making particles of a medium vibrate, creating compressions and rarefactions. Without particles to pass on the vibrations, sound cannot travel. This is contrasted with light and other electromagnetic waves, which can travel through a vacuum.

Solution:

1. The bell keeps vibrating and striking the gong the whole time, so the source of sound doesn't change.
2. As air is pumped out, fewer air particles remain inside the bottle to carry the vibrations to the glass walls and then to your ear.
3. With fewer particles, less energy is transmitted, so the sound gets fainter and fainter.
4. When nearly all the air is gone (a near-vacuum), the sound is almost inaudible.

Checking the options:

• A. Wrong. Wavelength depends on speed and frequency. It is not the reason the sound fades.
• B. Wrong. Sound passes through the bottle (glass) when air is present, so that isn't the issue.
• C. Correct. Sound needs a material medium, and removing the air removes the medium.
• D. Wrong. This is false. Sound cannot travel through a vacuum.

Answer: C. Sound needs a material medium.

Tip: For this bell jar question, remember "no medium, no sound." If you see options claiming sound travels in a vacuum, eliminate them immediately. Also note that the sound doesn't vanish at once because air is removed gradually, so the medium thins out slowly.