Physics
A hydrogen balloon floats up because of

Weight of the balloon is less than the weight of air displaced by it.
Air pressure decreases with decrease in weight
The pressure inside the balloon is more than the pressure outside it
Air pressure decreases with decrease in height

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Physics
Super conductors are substances that

Conduct electricity at low temperature
Offer high resistance to the flow of electric current
Offer minimum resistance to flow of electric current
Conduct electricity at high temperature

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