Cindy, a Real Estate Agent, Sells Her Friend Doug's House

Cindy, a real estate agent, sells her friend Doug's house without charging a commission. Weeks later in return Doug promises to give Cindy $1,000. This promise is not an enforceable contract because

A) the house has already been sold-the consideration is past.
B) selling the house was the agent's preexisting duty.
C) selling the house was a foreseen difficulty.
D) the promise of a payment of $1,000 is illusory.