On july $1$ of the current year, NSC company received an advance payment of $$6,000$ for work to be performer over the next $20$ months. This company's fiscal period is from January $1$ to december $31.$ The adjutung entry on December $31$ related to this transaction would involve a debit to $---$ and a credit to $----.$ The amount of the entry for the fees earned adjustment was $$---$