Power Systems
The advantage of hydro electric power station over thermal power station is

 hydro electric station can be constructed at the place where the energy is required
 the operation cost of hydro electric power station is low
 hydro electric power station can supply the power throughout the year
 the initial cost of hydro electric power station is low

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Power Systems
Critical mass is

  minimum mass of fissionable material required to sustain chain reaction.
 none of these.
 maximum mass of fissionable material required to sustain a chain reaction.
 a mass of fissionable material required to stop a chain reaction.

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