Power Systems
A sodium graphite reactors use

 a mixture of sodium and graphite as moderator.
  sodium as coolant and graphite as moderator.
 sodium as moderator and graphite as coolant.
 a mixture of sodium and graphite as coolant.

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Power Systems
In hydroelectric power plants

 both operating and initial cost are low.
  operating cost is low and initial cost is high.
 operating cost is high and initial cost is low.
 both operating and initial cost are high.

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