Answer:
See explanation section
Explanation:
(a) December 1   Cash       Debit   $18,000
            Unearned revenue    Credit   $18,000
Note: The company received the money in advance for a contract to do during December to April. Therefore, they received cash while a liability increased due to receiving advance money.
(b) December 31  Unearned revenue   Debit   $3,600
              Service revenue      Credit   $3,600
Note: As the company started performing, after the completion of 1st month, i.e., December 1 to December 31, the advance money started expiring because of providing services. Moreover, as the service is performed evenly for 5 months, the 1st month's revenue = $(18,000/5) = $3,600.