Physics
The threshold frequency is the frequency below which :

Photo current decreases with voltage
Photo current increases with voltage
Photo electric emission is not possible
Photo current is constant

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Physics
A stone floor feels cold to the bare feet but a carpet on the same floor feels warm. This is because

The human body maintains a constant temperature
The carpet is a better conductor of heat than the stone
The stone conveys the heat away from the feet more rapidly than the carpet
The floor and carpet are at different temperatures

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