Pablo was reviewing the water bill for his carwash business and determined that the highest bill, $ 5,300 occurred in July when 2,500 cars were washed and the lowest bill, $ 3, 200,occurred in February when 1,900 cars were washed. What was the variable cost per carwash?

A.$ 1.68

B.$ 1.28

C.$ 3.50

D.$ 2.12