(b) That's right. Sometimes, after a contract is made, circumstances change to make performance impossible or, if not impossible, then at least fundamentally different from what was envisaged when contracting. In such cases, it should be asked whether it can be inferred from the circumstances that the parties assumed the risk of the changed conditions. If they did not, and if it would be unjust to enforce the contract in the changed circumstances, then the contract is said to be frustrated. When frustration occurs, the contract is not made void, but it is treated as 'discharged'. This means that any contractual obligations that had not been performed prior to the frustration can no longer be enforced.
In the present case, although it is possible for the contract to be performed despite the changed circumstances, the cost of fuel has exceeded normal historical fluctuations. It is clear from what was said when contracting that neither A nor B assumed the risk of this happening. It is also strongly arguable that it would be unfair to enforce the contract in the changed circumstances. Accordingly, the contract will likely be treated as discharged by frustration.