Concept:Sound is a mechanical wave that requires a material medium such as air, water, or solids to travel.
Explanation:Inside the bottle, the electric bell produces sound waves.
These sound waves travel through the air present in the bottle.
As air is pumped out slowly, the amount of air molecules inside decreases.
With fewer molecules, the medium becomes less dense and cannot transmit the vibrations effectively.
Therefore, the loudness of the sound dies down gradually.
If all the air is removed to create a vacuum, no sound will be heard at all.
This is because sound cannot travel through a vacuum, since there is no material medium to carry its vibrations.
Answer:C. sound needs a material medium