Power Systems
Critical mass is

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

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Power Systems
An impulse turbine

 is most suited for low head and high discharge power plants.
 makes use of a draft tube.
  operates by initial complete conversion to kinetic energy.
 always operates submerged.

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