Which of the Following Would Be a Valid Offer

Which of the following would be a valid offer?

A) Bob's brand new riding lawn mower will not start,so Bob yells,"I'd sell this thing for $10."
B) Jane is a real estate agent,and while looking at a house she says,"I think we can get $50,000 for this place."
C) Mary goes to a garage sale,sees a dresser she wants,and says,"Would you take $25 for this dresser?"
D) John tells Rhoda,"I'll give you $500 for your motorcycle."