How often does the AdWords system run an auction to decide which ads to show on the Google search page

89) How often does the AdWords system run an auction to decide which ads to show on the Google search page

  A)  Every time a user enters a search query
  B)  Once every 24 hours for a given keyword
  C)  Every time a new advertiser adds a keyword to an account

  D)  Once every two hours for a given keyword