Power Systems
Critical mass is

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

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Power Systems
The draft tube is provided to

  increase the acting head on the water wheel.
 raise the water surface of the stream to create an artificial head.
 reduce the effect of water hammer.
None of these

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