please explain why the answer is D

ragmental Co. leased a portion of its store to another company for eight months beginning on October l atr nthly rate of $800. Fragmental collected the entire $6,400 cash on October 1 and recorded it as unearned revenue ssuming adjusting entries are only made at year-end, the adjusting entry made by Fragmental co.ot would be: 12) December 31 A) A debit to Rent Revenue and a credit to Cash for $2,400. B) A debit to Rent Revenue and a credit to Uncarned Rent for $2,400 C) A debit to Cash and a credit to Rent Revenue for $6,400. A debit to Unearned Rent and a credit to Rent Revenue for $2,400. ) A debit to Unearned Rent and a credit to Rent Revenue for $4,000