The Opportunity Cost of an Action

The opportunity cost of an action:

A) can be objectively determined only by economists.
B) is a subjective valuation that can only be determined by the individual who chooses the action.
C) can be determined by adding up the bills incurred as a result of the action.
D) can be determined by considering both the benefits that flow from as well as the monetary costs incurred as a result of the action.