How can Under Consumption come due to Ignoring Positive Consumption Externalities? -What is the Welfare Gain

-Let MPC = MSC. But MPB have Different Line to MSB -Socially Optimal Point is when MSB = MSC. [Point A] -Free Market Point will be when MPB = MPC [Point B] -There is an Under Consumption as MSB is further Right than MPB, and the Price is Lower than what Society wants -Welfare Gain [Gain to Society LOST by ignoring Externality] will be Point A, Point B, and the External Benefit