(b) That's wrong. To discharge their contractual obligations, the parties must do what was promised (specified) in the contract. Specific performance simply means performance in the specified terms. If what is sold is 'my dog Rover', then that particular dog must be delivered in order to discharge the contract. There is legislation that sets out the requirement that a seller of goods must deliver the goods as identified in the contract.
In the present example, the parties have specified what is being bought and sold by referring only to the category or type of goods - wheat. This is known as a sale of generic goods, because the goods are identified by their class (genus). In such circumstances, the seller's obligation is only to deliver goods belonging to the specified class. In the present case, it does not matter that the wheat comes from Canada: A has supplied the agreed goods within the promised time.
Note: A and B have not specified what quality the wheat should have but the relevant sale of goods legislation would require that, in these circumstances, the wheat supplied be of merchantable quality (see the module Contents of a Contract).