Physics
In the gamma decay of a nucleus

The mass number of the nucleus does not change whereas its atomic number changes
Neither the mass number nor the atomic number of the nucleus 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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Physics
On a cold day when a room temperature is 15oC, the metallic cap of a pen becomes much colder than its plastic body, though both are at the same temperature of 15oC, because

plastics have a lower density than metals
plastics have a higher thermal conductivity than metals
metals have higher thermal capacity than plastics
metals are good conductor of heat

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