What Is the Optimal Supply of Roadside Litter

What is the optimal supply of roadside litter?

A) Zero,because it is costless for each individual to throw away his or her own trash.
B) The quantity of litter that would remain if trash were picked up until the average cost of picking up trash equaled the total benefit.
C) The quantity of litter that would remain if trash were picked up until the marginal cost of picking up trash equaled the marginal benefit.
D) The quantity of litter that would remain if trash were picked up until the total cost of picking up trash equaled the marginal benefit.