the coreect answer is A, but why?

a 9) At the end of the year, Mark Inc, estimates future bad debts to be $6,500. The Allowance for Uncollectible Accounts has a credit balance of $2,500 before any year-end adjustment. What adjustment should Mark Inc. record for the estimated bad debts at the end of the year? A) Debit Bad Debt Expense, $6,500; credit Allowance for Uncollectible Accounts, $6,500. Bad D. B) Debit Bad Debt Expense, $4,000; credit Allowance for Uncollectible Accounts $4,000. C) Debit Allowance for Uncollectible Accounts, $9,000; credit Bad Debt Expense, $9,000. 6500 D) Debit Bad Debt Expense, $9,000; credit Allowance for Uncollectible Accounts, $9,000