Which of the Following Is True with Regard to Ending

Which of the following is true with regard to ending a contract by agreement?

A) A contract can be ended by agreement if one party has performed his obligation and agrees to take something as satisfaction from the other although that other hasn't performed according to the contract.
B) A contract can be ended by one party if he finds a better deal (e.g., by cancelling an order that has been accepted) as long as this is done before performance.
C) A contract cannot be ended by the happening of a "condition subsequent" provided for in the contract.
D) A contract can be ended unilaterally by one party.
E) A contract cannot be ended by agreement if neither party to the contract has performed his or her obligation.