Answer:
B) an unforeseen difficulty supported the contract modification here.
Step-by-step explanation:
Sometimes unforeseen circumstances may change a contract and other times they might even dismiss a contract.
Usually when parties sign a contract an Unforeseen Events Clause is included. That clause usually states that neither party can be liable for losses or damages caused by unforeseen events that are beyond any of the party's reasonable control.
For example, I have a contract to provide a service in San Francisco, but suddenly an earthquake destroys the part of the city where my hotel was located and I'm forced to leave the city without performing my obligations. The other party cannot sue me for breaching the contract since the earthquake was beyond my control and knowledge.