Physics
Siphon will fail to work if:

The level of the liquid in the two vessels are at the same height.
The temperature of the liquids in the two vessels are the same.
The densities of the liquid in the two vessels are equal.
Both its limbs are of unequal length.

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Physics
In the gamma decay of a nucleus

Neither the mass number nor the atomic number of the nucleus changes
The mass number of the nucleus does not change whereas its atomic number changes
Both the mass number and the atomic number of the nucleus change
The mass number of the nucleus changes whereas its atomic number does not change

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