Carnival Co. uses aging of accounts receivable method. At the end of year 2011, Carnival estimated that it needed $500,000 in allowance for doubtful account. Carnival had a previous credit balance of $100,000 in allowance for doubtful account from 2010. What should be the bad debt expense for 2011?

A) $500,000

B) $900,000

C) $400,000

D) $600,000