Ron Moss, a manager of Waterworks, Inc, was reviewing the water bills of a dog daycare and spa. He determined that its highest and lowest bills of $3,600 and $2,800 were incurred in the months of May and November, respectively. If 500 dogs were washed in May and 200 dogs were washed in November, what was the variable cost per dog associated with the company's water bill? (Round your answer to the nearest cent.)

A) $4.00
B) $14.00
C) $7.20
D) $2.67

D .D) Variable cost per unit = Change in total cost / Change in volume of activity
Variable cost per unit = ($3,600 - $2,800 ) / (500 dogs - 200 dogs) = $800 / 300 dogs
Variable cost per unit = $2.67 per dog