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

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Physics
Isotopes of an element contain

the same number of protons but different number of neutrons
equal number of protons and electrons
the same number of neutrons but different number of protons
equal number of nucleons

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