Physics
Why does the sea appear blue in colour?

Interference of the sun light from water surface
Scattering of the sunlight by water molecules
Reflection of the sun light from water surface
Refraction of the sun light

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Physics
In a railway track, two rails are joined end to end with a gap in between them because

Accidents due to expansion in summer can be avoided
Accidents due to contraction in winter can be avoided
Air-gaps are necessary for bearing the weight of running train
Steel can be saved

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