please explain!!

Sales is $120 per unit; direct material is $21 per unit; direct labor is $9 per unit, overhead is $6 per unit and $9,000 per month; selling and administrative expenses are $12 per unit and $18,000 per month. The company produced 1,000 units. Total period costs are: $36,000 $18,000 $30,000 $45,000