What key issue should be addressed?

A retail company’s database of record resides at a 3rd-party company that also keeps track of purchase history.

That database only updates once a day where new records can be created and merged.

The database uses an "Email ID," which is a numeric field that represents both the business unit and email address.

The company now wants to be able to send real-time Welcome emails to people (newly registered website users) who provide their email address in exchange for getting 10% off their first order, and ensure this send is connected to "Email ID" in the database.

What key issue should be addressed? Choose 3 answers
A . How will Marketing Cloud and the database sync up?
B . Will new users have an "Email ID"?
C . What will be used as Subscriber Key?
D . What publication lists will be used?
E . Will the company need a custom preference center?