can someone explain me this?

On January 1, All City Services has the following balances Accounts Receivable $22,000 Bad Debts Expense $0 All City has the following transactions during January: Credit sales of $120,000, collections of credit sales of $84,000, and write-offs of $18,000. All City uses the direct write-off method. The amount of Bad Debts Expense for January is O A. $12,600 O B. $25,714 O C. $22,000 O D. $18,000