Rible Company has observed that at an activity level of 8,000 units the cost for maintenance is 15,000, and at 10,000 units the cost for maintenance is 16,500. Using the high-low method, the cost formula for maintenance is:

1.875 per unit.

15,000 plus 0.15 per unit.

1.65 per unit.

9,000 plus 0.75 per unit.