Sedan Company provided services worth $4,000 to its customer in April and billed the customer at the time. During the current month in May, the customer paid off the balance owing in full. Which of the following journal entries is required to record the transaction in May? 

 a. Debit Accounts Receivable; credit Service Revenue 

b. Debit Accounts Receivable; credit Unearned Revenue

c. Debit Cash; credit Accounts Receivable 

d. Debit Cash; credit Service Revenue