In December, services were performed for a total of $1,500 but no bill has been sent to the customer as yet and no record has been made in the accounting system. The adjusting entry to be made is:

a. a debit to Unearned Revenue and credit to Accounts Receivable for $1,500

b. a debit to Accounts Receivable and credit to Unearned Revenue for $1,500

c. a debit to Accounts Receivable and credit to Service Revenue for $1,500

d. a debit to Unearned Revenue and credit to Accounts Payable for $1,500