Physics
A hydrogen balloon floats up because of

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

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Physics
It is easier for a man to swim in sea water, than in river water because

The density of sea water is lesser than the density of river water
The density of sea water is more than the density of river water
The sea is-deeper than the river
The sea is wider than the river

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Physics
The sky appears blue because

It is actually blue
The atmosphere scatters blue colour more than the others
All colours interfere to produce blue
In white light, the blue component dominates

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