1. $7,000 of merchandise inventory was ordered on September 2, 2009 2. $3,000 of this merchandise was received on September 5, 2009 3. On September 6, 2009, an invoice dated September 4, 2009, with terms of 3/10, net 30 for $3,250 which included a $250 prepaid freight cost, was received. 4. On September 10, 2009, $800 of the merchandise was returned to the seller. Based on the above information, by what date does the invoice need to be paid in order to take the advantage of the discount?

a. September 15, 2009
b. September 16, 2009
c. September 10, 2009
d. September 14, 2009