Which of the following pricing strategies best approximates the marginal cost of a service? Provide a brief justification for each answer.

a. $18.20/thirty days for residential solid-waste service or variable rate by container size

b. Water rates that combine a facility charge, or wastewater rates based on 98 % of volume charge during wintertime usage

c. Airport parking fees based only on length of stay, or parking fees based on length of stay by proximity of lot to terminal

d. Tuition rate that are the same for fall, spring, and summer, or tuition rates that are higher for fall and spring than for summer