Could I have only answer? If I need a explaination, I will leave a additional comment. Thanks.

The default rule that required agents to perform tasks in contracts that became impossible was:

1) An inefficient rule that fails to reduce uncertainty.

2) An efficient rule that reduces transaction costs.

3) A rule that replaces the foreseeability standard for breach of contract.

4) An inefficient rule that has been corrected by statute in most of Canada.