Hello I needed help with this problem. It is a trick question. Please show work for each question.

Price is $20 per unit. At current sales volume, unit cost (total cost per unit) is $15. It consists of $5 per unit of fixed costs and $10 per unit of variable costs.

Question: How much will the profit change if we sell one additional unit?

A. increase by $5

B. increase by $10

C. increase by $15