3. Two people board an elevator on the fourth floor. The door stays open for a while after they board. One of them, A, pushes the button for floor ten, and the other, B, pushes the button for fourteen. The door opens at six and one more person, C, gets on. She pushes the button for eight and gets off there.

A gets off at ten, and B gets off at fourteen. The elevator was never crowded and no one was intending to pick their nose during the trip. There was a person waiting on fourteen to go down. No one else was waiting for the elevator.

The elevator passengers all weigh less than 300 pounds, and all of them have time that is about average in value among educated New York City residents. In terms of social cost, whose trip was the most costly?

Whose was the least? Why?