What should a Consultant recommend to meet this requirement?

Cloud Kicks’ VP of Technology wants to start using Salesforce for all of the sales teams automation. 70 million records were all migrated from a legacy database to the data warehouse that will be synced with Sales force. Cloud Kicks wants to be able to search and cross-reference records with the original source database .

What should a Consultant recommend to meet this requirement?
A . Use a custom External ID field and map this to the original record ID value
B . Use the standard External ID field and map this to the original record ID value.
C . Use a custom field named External ID and map this to the current record ID value.
D . Use the standard External ID field and map this to the current record ID value.

Answer: A