Physics
Plants are dried up in winter due to frost because

There is mechanical loss of tissues and drought condition prevails
Respiration does not take place at low temperature
Photosynthesis does not take place at such low temperature
Evaporation does not take place

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Physics
On a cold day when the room temperature is 15°C, the metallic cap of a pen becomes much colder than its plastic body, though both are at the same temperature of 15°C, because :

Plastics have a lower density than metals
Metals have higher thermal capacity than plastics
Metals are good conductors of heat
Plastics have a higher thermal conductivity than metals

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Physics
When a piece of pure silicon is doped with aluminium, then

The doped silicon piece will become n-type
The doped silicon piece will become p-type
The conductivity of the doped silicon piece will remain the same
The resistivity of the doped silicon piece will increase

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