7. In January 2021 , Thompson Company received a $ 36,000 customer prepayment for services to be provided in 2021 and 2022. The services were performed equally over the two years . The $ 36,000 was reported as service revenue in 2021 , even though the services were performed as scheduled. Assuming 2021 is now closed but 2022 is still open , what correcting entry is needed ? A, B, C, or D?

\begin{tabular}{|l|r|r|} \hline & Debit & Credit \\ \hline Unearned revenue & 18,000 & \\ \hline Retained earnings & & 18,000 \\ \hline & Debit & Credit \\ \hline Retained earnings & 18,000 & \\ \hline Unearned revenue & & 18,000 \\ \hline \hline & Debit & Credit \\ \hline Retained earnings & 18,000 & \\ \hline Service revenue & & 18,000 \\ \hline \hline & Debit & Credit \\ \hline Unearned revenue & 18,000 & \\ \hline Service revenue & & 18,000 \\ \hline \end{tabular}