Power Systems
Non-commercial sources of energy are

None of these
 fossil fuel, hydro power and nuclear power.
  wood, animal wastes and agricultural wastes.
 solar, wind and biomass.

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

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

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