(a) That's wrong. Sometimes what appears to be a single contract turns out on proper analysis to be divisible into two or more separate contracts. This can be important because if two separate contracts exist, a failure to properly perform according to the terms of one of these smaller contracts does not affect the other.
If A's promise to repair the roof of B's house and his promise to paint it are properly regarded as divisible contracts, then the failure to paint the roof does not prevent A from claiming payment for the completed repairs. But if the repairs and painting are regarded as a single, indivisible undertaking by A, then he has not completed the whole job and cannot claim payment.
The facts in the case-study suggest that there was a single, indivisible contract between A and B. Although the work consists of first doing repairs and then painting, it is clear that the two tasks were closely related and payment was promised only when both tasks were completed. The best conclusion in these circumstances is that A has failed to perform one indivisible agreement and therefore is not entitled to counter-performance from B.