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

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

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Physics
What is a compound microscope?

A microscope whose lenses are concave.
A microscope that has one lens.
A microscope that has two sets of lenses : an occular lens and an eyepiece.
A microscope whose lenses are convex.

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