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.