Suppose that a business sells 6-month subscriptions to its monthly magazine. On January 1, the company receives a total of $600 for 10 subscriptions. To record this transaction, the company debits "Cash" for $600 and credits "Unearned Subscription Revenue" for $600. As of January 31, the company has provided one month of magazines and has earned one month of revenue. What adjusting entry is necessary at January 31?