Which information does the Conversion Optimizer need in order to find the optimal cost-per-click (CPC) bid for an ad each time the ad is eligible to appear?

Which information does the Conversion Optimizer need in order to find the optimal cost-per-click (CPC) bid for an ad each time the ad is eligible to appear?

  A)  Manual bid changes
  B)  Historical conversion data
  C)  Test conversions
  D)  Forecast data