Which of the following is correct?

A contract is frustrated when something happens, after it has been entered into, which renders the contract more difficult to perform.

A contract is frustrated when something happens, after it has been entered into, which renders the contract impossible to perform.

A contract is frustrated when a party expressly agrees to manufacture and supply goods and then discovers that they will be far more expensive to produce than they thought at the time of the contract.

A contract is frustrated if it is impossible to perform at the time that it is made.