Redline Editorial Services sent a $1,000 invoice to a customer for copyediting a booklet. According to the terms of the invoice, the customer would receive a 3 percent discount on the invoice price if the invoice was paid within 15 days. This is an example of a:

a. Quantity discount.
b. Cash discount.
c. Rebate.
d. Functional discount.
e. Promotional allowance.