At an activity level of 6,000 units, the cost for maintenance is $7,200; at 10,000 units, the cost for maintenance is $11,600. Using the high-low method, what is the cost formula for maintenance? 1) $1.16 per unit. 2) $1,200 plus $1.10 per unit. 3) $1.20 per unit. 4) $600 plus $1.10 per unit.