At an activity level of 6,000 units the cost for maintenance is $7,200 and at 10,000 units the cost for maintenance is $11,600. Using the high-low method, the cost formula for maintenance is: a. $1.20 per unit b. $1.16 per unit c. $1,200 plus $1.10 per unit d. $600 plus $1.10 per unit