Physics
The law of conservation of energy states that

Energy can neither be created nor destroyed
Energy can either be created or destroyed
Energy can neither be created or destroyed
Energy can either be created nor destroyed

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

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