(b) That's right. When a breach of contract happens, the non-defaulting party is not entitled to sit back and take maximum advantage of the right to claim damages. Instead, the non-defaulting party is required to do everything reasonably possible to mitigate (minimise or reduce) the immediate and consequential losses that flow from the breach. Exactly what must be done will differ from case to case. The non-defaulting party is not obliged to take risks, or spend money they cannot afford, in an effort to mitigate the losses. In the circumstances, it seems reasonable that A should have purchased the seeds he needed from another supplier and proceeded with planting. B must prove that A has failed to take such reasonable steps. If B succeeds in this, A's claim for damages will be limited to the part of the loss that was unavoidable.
Note: In the case of an anticipatory breach of contract, the non-defaulting party is given a choice by law whether to terminate performance or not. This choice can be made freely, regardless of whether the option chosen results in a higher claim for damages than the alternative would have.