The cost of renting a snow blower is $13.50 per hour. What is the rule?
a. C = 13.50 + x, where C is cost and x is one hour of renting a snow blower
b. C = -13.50 -x, where C is cost and x is one hour of renting a snow blower
c. C = 13.50 * x, where C is cost and x is one hour of renting a snow blower
d. C = -13.50 ÷ x, where C is cost and x is one hour of renting a snow blower