For each transaction below, calculate the amount of revenue to be recognized in the current period using accrual-basis accounting:

a. Performed $21,000 of services during the month and received full cash payment from customers at the time of service.

b. Performed $10,000 of services during the month and billed customers. Customers are expected to pay next month.

c. Received $16,000 cash from customers for services to be provided next month.