Shirley, Inc. records its bad debt expense using the credit sales method. Total sales during 2021 were $1,700,000, which consisted of $400,000 of cash sales and the remainder were credit sales. Shirley's estimation is that 4% of net sales would be uncollectible. During the year, Shirley wrote off $150,000 of accounts receivable and the allowance for doubtful accounts amount is $75,000. What is the amount of bad debt expense for 2021?

a. $55,000

b. $52,000

c. $28,000

d. $6,000