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 glass of the bell jar absorbs the sound waves
The bell jar is too small

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Physics
Isotopes of an element contain

equal number of nucleons
equal number of protons and electrons
the same number of neutrons but different number of protons
the same number of protons but different number of neutrons

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