Physics
The ringing of an electric bell in an enclosed evacuated glass bell jar is not heard outside because

The ringing is not loud enough
The bell jar is too small
There is no air or any other gas in the bell jar
The glass of the bell jar absorbs the sound waves

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Physics
A hydrogen balloon floats up because of

Weight of the balloon is less than the weight of air displaced by it.
Air pressure decreases with decrease in height
Air pressure decreases with decrease in weight
The pressure inside the balloon is more than the pressure outside it

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