Which of the Following Situations Best Illustrates the Rule That

Which of the following situations best illustrates the rule that a promise to perform a pre-existing public duty does not constitute consideration?

A) Pam has threatened to sue Dave for $10 000.She is quite sure that she would win the case in a public court.However, because she is very fond of him, she has also generously offered to settle the matter for $6000.
B) Pam, who is a fire fighter employed by the city, agreed to extinguish a fire that was destroying Dave's house, which is located in the city, only after he promised to pay $10 000 to her in exchange for her services.
C) Pam placed an advertisement in a newspaper that promised to pay $10 000 to anyone who returned her lost cat to her.
D) In exchange for his promise to pay $5, Pam promised Dave that she would guard a parking lot in which his car was located during a sporting event.Several minutes later, she promised Earl, who had also parked there, that she would guard the same parking lot during the same sporting event if he promised to pay her $7.
E) Pam asked Dave, who is a real estate agent, to locate a house in which she could operate a small accounting business.After he directed her attention to such a house, she promised to pay $10 000 to him.