The Terms on an Invoice Are 4/15, N/30

The terms on an invoice are 4/15, n/30. This means that

A) a discount of 4% will be allowed if payment is made within 15 days from the date of the invoice.
B) a discount of 15% will be allowed if payment is made with 4 days from the date of the invoice.
C) a discount of 4% will be allowed if payment is made by the 15^th of the month.
D) a discount of 15% will be allowed if payment is made by the 4^th day after the date of the invoice.