Susan wishes to buy gasoline and have her car washed.She finds that if she buys 9 gallons of gasoline at $2 per gallon,the car wash costs $1.50,but if she buys 10 gallons of gasoline,the car wash is free.For Susan,the marginal cost of the tenth gallon of gasoline is:_______________.
a. 50 cents.
b. $1.
c. zero.
d. $1.50.