(b) That's right. A has contracted to supply a specified grade of wheat at a fixed price. At the time agreed for delivery, that price is below the market price. If A fails to perform his obligations, B will have to buy wheat from another supplier at a higher price. You might expect the courts would order specific performance in these circumstances, but in fact they would not. Remember, specific performance is only ordered if damages are not an adequate remedy.
In the present case, B can readily obtain equivalent goods from another supplier, and if she pays more for it, she can claim the difference as damages from A. This is clearly a situation where damages are an adequate remedy and specific performance would not be ordered. Compare the case below.