Marginal Costs

79. A driver wishes to buy gasoline and have her car washed.  She finds that the wash costs $3.00 when she buys 19 gallons at $1.00 each, but that if she buys 20 gallons, the car wash is free.  Thus the marginal cost of the twentieth gallon of gas is:
A) -$2.00.
B) $0.00.
C) $1.00.
D) $2.00.
E) none of the above.