If the cost-per-thousand impressions (CPM) option is not available for a campaign, the most likely reason is that the campaign:

69) If the cost-per-thousand impressions (CPM) option is not available for a campaign, the most likely reason is that the campaign:

  A)  has used CPM pricing before
  B)  is only opted into the Google Display Network
  C)  has never used CPM pricing before

  D)  is only opted into the Google search and the Search Network