Power Systems
Half life is defined as

 the time required for full of atoms in a specimen disintegrate.
  the time required for one half of isotope to decay.
 none of above.
 the time required to spiliting of a heavy nucleus.

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

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