I need detailed help with steps on how to solve : and each step should be explained in detail with the concepts and also label calculations and show how final answer is attained (i.e. what numbers are added/subtracted/divided/multiplied to get the result)

Roberts Company produces a single product. This year, the net operating income was $2,000 lower under absorption costing than under variable costing. The company sold 8,000 units during the year, and its variable costs were $8 per unit, of which $2 was variable selling and adminstrative expense. If production cost was $10 per unit under absorption costing, how many units did the company sell during the year?

A) 7500 B) 7000 C) 9000 D) 8500 E) None

Thanks :)