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

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Physics
X-rays region lies between

Visible and infra-red region
Gamma-rays and ultraviolet region
Ultraviolet and visible region
Short radio waves and long radio waves

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