Physics
Which of the following are used for accurately measuring very small time intervals ?

Quartz clocks
Atomic clocks
White dwarfs
Pulsars

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

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
Steel can be saved

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