Gulliver's Groceries Purchased Milk Cartons at an Invoice Price of $9,000

Gulliver's Groceries purchased milk cartons at an invoice price of $9,000 and terms of 2/10, n/30. On arrival of the goods, Gulliver's realized that half of the milk was past the expiration date, and returned them immediately to the supplier.
If Gulliver's pays the remaining amount of the invoice within the discount period, the amount paid should be:

A) $4,320
B) $8,820
C) $9,000
D) $4,410