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
There is no air or any other gas in the bell jar
The bell jar is too small
The glass of the bell jar absorbs the sound waves

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Physics
Inside an aeroplane, flying at a high altitude,

normal humidity and partial vacuum are maintained.
the pressure inside is less than the pressure outside.
normal atmospheric pressure is maintained by the use of air pumps.
the pressure is the same as that outside.

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Physics
When a pail of water is swung in a vertical circle, the water does not fall out at the top of the loop when the speed is

Below a certain minimum value irrespective of amount of water in the pail
Below a certain minimum value depending on amount of water in the pail
Above a certain minimum value depending on amount of water in the pail
Above a certain minimum value irrespective of amount of water in the pail

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