fire extinguisher are used to stop the fire . in fire extinguisher carbon dioxide is present . fire can be extinguished by water also but not in the purpose of electrical equipment because water is a conductor of electricity therefore carbon dioxide is used widely as fire extinguisher .fire extinguisher where carbon dioxide is present inside helps to remove 1 of the important source which leads to the creation of fire and that is the oxygen . it makes a layer of carbon dioxide so that oxygen can't reach the fuel and as a result fire stops out burning . 
  i hope that this will help u..........................^_^
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and it did help me
ur welcome friend ^_^
Extinguisher means to repose or to stop... Therefore fire extinguisher are the tools with whom we can get control on fire .... They basically have CO2 carbon dioxide as there basic component as CO2 stops fire