MBP = MAC and MCP are the marginal benefit (marginal abatement cost) and marginal cost of pollution curves for farmers’ fertilizer use. The efficient level of water pollution is quantity POLL* of fertilizer runoff. If government regulators incorrectly estimate farmers’ marginal abatement costs at MAC_e < MAC, the quantity-based intervention would reduce pollution to POLL_B < POLL*, while the Pigouvian tax would increase pollution to POLL_C > POLL*. Because MAC is flat relative to MCP, the quantity-based intervention is preferable to the price mechanism, as seen by comparing the resulting deadweight losses from the two interventions (X < Y).