What is the implication of this data model when using Marketing Cloud Connect?

A customer wants to use Sales Cloud as a system of record for email messages sent from Marketing Cloud; however, the customer only sends from custom objects and cannot use the Contact ID or Lead ID as the Contact Key in Marketing Cloud.

What is the implication of this data model when using Marketing Cloud Connect?
A . Email Sends will fail if the Contact ID or Lead ID is not included.
B. Tracking Data will not be returned to the Sales Cloud email recipient.
C. The customer will be able to use Reports and Campaigns as audiences.
D. The customer will be unable to use synchronized data extensions.

Answer: A