In Which of the Following Situations Can a Party's Duty

In which of the following situations can a party's duty to perform be discharged?

A) Statutes and legislation that make a performance more difficult.
B) When something that is essential to the promisor's performance is destroyed through no fault of the promisor but its substitute is available.
C) When events occur after the formation of the contract that would make the return performance of the other party worthless to the promisor.
D) When the promisor impliedly assumed the risk that the event would occur.