Under what circumstances would it be efficient to charge a positive price for drivers to cross the Champlain Bridge in Montreal or the Lion's Gate Bridge in Vancouver? O A. when it is possible to exclude drivers from the bridge O B. when congestion on the bridge means that the marginal cost of providing access to one more driver is positive O C. when the marginal private benefit to the last driver is greater than the marginal private cost to that driver O D. when it is not possible to exclude drivers from the bridge O E. when the sum of marginal benefits to all drivers is positive