How can the Marketer most easily achieve this during creation of the data extension?

Northern Train Outfitters needs to ensure that "Valued Customer" appears if no data exists in that field for a subscriber when using a personalization string to display the First_Name field in a data extension.

How can the Marketer most easily achieve this during creation of the data extension?
A . Ensure the Nullable box is not checked for the First_Name field
B. Fill in the source data with "Valued Customer" for each blank First_Name field
C. Mark First_Name as the Primary Key
D. Make "Valued Customer" the Default Value for the First_Name field

Answer: D