In Caymus Company’s December 31 trial balance, a credit balance of $31,500 appears in Unearned Fee Income. This amount represents cash received from a customer on November 1 covering work to be performed by Caymus in November through January. At December 31, Caymus had earned $10,500 of the amount received on November 1.  Which of the following would be used to adjust the Unearned Fees Income account on December 31?
a. Debit Unearned Fee Income for $31,500.
b. Debit Unearned Fee Income for $20,000.
c. Debit Fees Income for $10,500.
d. Credit Fees Income for $10,500.