Which of the following partner functions can cause this problem?

In a standard SAP S/4HANA system, you have create a sales order. Upon saving, you receive a system message that a business partner of a specific partner function is missing.

Which of the following partner functions can cause this problem? Note: There are 2 correct answers to this question.
A . Forwarding agent
B . Contact person
C . Bill-to-party
D . Payer

Answer: C,D