The cost, in dollars, of a one-day car rental is given by C(x) = 31 + 0.18x, where x is the number of miles driven. In this function,
A) $0.18 is the cost per day to rent the car and $31 is the cost per mile.
B) $31 is the cost per day to rent the car and $0.18 is the cost per mile.
C) $18 is the cost per day to rent the car and $0.31 is the cost per mile.
D) $31 is the cost per day to rent the car and $18 is the extra cost for unlimited miles.