Which of the Following Considerations Is Legally Insufficient

Which of the following considerations is legally insufficient?

A) Tony bought a book for $100. Ruby offers to buy it for $10.
B) Tony bought a text book for $100. He offers to give it to Ruby if Ruby helps him move into his new dorm room.
C) Tony bought a book for $1,000. Ruby offers to buy it for $2,000 if it does not rain the next day.
D) Tony bought a book for $100. He offers to sell it to Ruby for free if she helps him befriend the new girl in their dorm.
E) None of these is insufficient consideration.