the correct answer is D, Could any one explain it to me please?

Question: Opportunity Costs in the Make versus Buy Decision Richardson Co. uses ten units of T305 each month. The cost to manufacture one unit of T305 is shown below. Material handling, which is not included in manufacturing overhead, represents the direct variable costs of the Receiving Department that are applied to direct materials and purchased components on the basis of their cost. Richardson's annual manufacturing overhead budget is one-third variable and two. thirds fixed. Simpson Castings has offered to supply T305 at a unit price of $30,000. Assume there is no rental opportunity and Richardson could use the idle capacity to manufacture another product that would contribute $104,000 per month. If Richardson chooses to manufacture the ten T305 units in order to maintain quality control, Richardson's opportunity cost is: a. $(96,000) Direct materials $2,000 b. $(56,000) c. $(40,000) Material handling (20% of direct material cost) 400 d. $8,000 Direct labor 16,000 Manufacturing overhead (150% of direct labor) 24,000 Total manufacturing cost $42,400