The Hydro water department has a monthly service charge of $15.40 and a volume charge of $1.05 for every 100 cubic feet of water. The total monthly cost is equal to the service charge plus the charge for the volume used during the month. If we let x represent the number of cubic feet of water (in hundreds) and y represent the total monthly cost, then we have: total monthly cost = volume charge + service charge y = 1.05x + 15.40 If the Johnson family used 300 ft3 less water this month than last month, then they can expect A. an increase in their water bill of $46.20. B. an increase in their water bill of $3.15. C. a decrease in their water bill of $3.15. D. a decrease in their water bill of $46.20.