Power Systems
The commercial sources of energy are

 solar, wind, biomass.
 none of above.
 wood, animal wastes and agricultural wastes.
  fossile fuels and radioactive substances.

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Power Systems
Critical mass is

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

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