What material is used for natural gas piping?

What material is used for natural gas piping?
 
  A) Aluminum tubing B) Galvanized pipe C) Iron pipe D) Copper tubing

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[Ques. 2] The purpose of the condenser in a refrigeration system is to:
 
  A) Reject heat from the refrigerant.
  B) Absorb heat into the refrigerant.
  C) Increase the pressure and temperature of the refrigerant.
  D) Reject heat from the condensing medium.

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[Ques. 3] What material is used for refrigerant piping heating, air-conditioning, and refrigeration systems?
 
  A) Aluminum tubing B) Galvanized pipe C) Iron pipe D) Copper tubing

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[Ques. 4] Compressor cylinder unloading reduces the capacity of a reciprocating compressor by:
 
  A) Making the valves in the head inoperative.
  B) Disconnecting the pistons in the unloaded cylinders.
  C) Flooding the unloaded cylinders with oil.
  D) Blocking off the discharge port in the unloaded heads.

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[Ques. 5] The refrigerant in a mechanical compression refrigeration system absorbs and releases heat by:
 
  A) Changing its physical state. B) Changing its temperature.
  C) Changing its molecular composition. D) Converting energy to mass.