What is the difference between the SAP sales documents Delivery Free-of-Charge and Subsequent Delivery Free-of-Charge?

What is the difference between the SAP sales documents Delivery Free-of-Charge and Subsequent Delivery Free-of-Charge?
A . Subsequent Delivery Free-of-Charge requires a mandatory reference to a sales document. Delivery Free-of-Charge does not have this requirement.
B . Subsequent Delivery Free-of-Charge uses item category KLN, which carries out no pricing and no billing. Delivery Free-of-Charge uses item category TAN, which carries out pricing and billing.
C . Delivery Free-of-Charge must be created with reference to a preceding invoice. Subsequent Delivery Free-of-Charge can be created with reference to an invoice or sales order.
D . Subsequent Delivery Free-of-Charge is only created in case of partial deliveries. Delivery Free-of-Charge is created with reference to return deliveries only.

Answer: A