(a) That's right. The agreement is that A will supply coffee beans and B will pay for them. In this type of case, the duties owed by the parties are reciprocal - the performance by one party is done in expectation that the other party will counter-perform.
When there are reciprocal duties of performance it is presumed that the parties intended that they would both perform at the same time. For example, when goods are sold, the sale of goods legislation provides that payment must be made at the same time as delivery. When services are rendered, payment is due when the work is completed. The presumption applies unless the agreed terms provide otherwise (such as when goods or services are supplied on credit terms).
So, in the example, A is certainly entitled to payment at the same time as handing over the coffee beans and B could not demand performance from A without first tendering payment.