Power Systems
In a pressurized water reactor

 the coolant water boils in the reactor core.
  both B and C.
 heavy water is used as both coolant and moderator.
 water is used at high pressure so that it is available is liquid from at about 280° C.

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Power Systems
Gross head of an hydroelectric power station is

  the difference of water level between the level in storage and tail race.
 the height of water level in the river where the tail race is provided.
 all of above.
 the height of water level in the river where the storage is provided.

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