Which of the Following Situations Most Likely Illustrates the Concept

Which of the following situations most likely illustrates the concept of pre-existing public duty not constituting consideration?

A) Pam has threatened to sue Dave for $10 000.She is quite sure that she would win the case in court.However,because she is very fond of him,she has also generously offered to settle the matter for $6000.
B) 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.
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,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.