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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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
It provides oxygen to a drowning person
The person can sit on it like a raft

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