Refer to the figure below. The socially optimal quantity in this market could be achieved by imposing a ______ equal to the vertical distance ______. 
Definitions:
Variable Selling Cost
Variable selling costs fluctuate with sales volume; they include expenses such as commission and shipping that increase with higher sales.
Constrained Resource
A limited resource within a system that significantly affects the throughput or output of the system.
Special Order
A special order refers to a one-time purchase order often at a discounted price, usually for large quantities that is outside the normal scope of operations.
Direct Labor Hours
Total working hours logged by employees who are intimately involved with the production operations.
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