At 15 units of the activity, marginal benefit is $______ and marginal cost is $______. b. Adding the 15th unit of activity causes net benefit to __________ (increase, decrease) by $______. c. At 35 units of the activity, marginal benefit is $______ and marginal cost is $______. d. Subtracting the 35th unit of activity causes net benefit to __________ (increase, decrease) by $______. e. The optimal level of activity is ______ units, MB = $______ and MC = $______. f. Can you compute total benefit, total cost, and net benefit for the optimal level of activity? If so, how? If not, why not?