Why is the answer D?

Question:

Rusty Hardware makes only cash sales. It began 2018 with a credit balance of $32,000 in the refund liability account. Sales during 2018 were $600,000. Rusty estimates that 6% of all sales will be returned. During 2018, customers returned merchandise for credit of $28,000 to their accounts.

Rusty's 2018 income statement would report net sales of:

A) $568,000

B) $604,000

C) $600,000

D) $564,000

The answer is D, but why? Can you explain this conceptually. I thought we were to take the gross revenue, which is $600,000 and subtract it by estimated allowances and actual sales returns, thus by $36,000 and by $28,000