At year-end, Chief Company has a balance of $18,000 in accounts receivable of which $1,800 is more than 30 days overdue. Chief has a credit balance of $180 in the allowance for doubtful accounts before any year-end adjustments. Using the aging of accounts receivable method, Chief estimates that 1.0% of current accounts and 9% of accounts over thirty days are uncollectible. What is the amount of bad debt expense?

$144.

$180.

$324.

$504.