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
What is the principle of a ''Life Jacket"?

It decreases the volume of the person to keep him afloat
It increases the volume of the person to keep him afloat
The person can sit on it like a raft
It provides oxygen to a drowning person

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