What should a consultant recommend meeting this requirement?

Cloud Kicks’ (CK) VP of technology wantsto start using Salesforce for all the sales team’s automation. CK migrated 70 million records from a legacy database to the datawarehouse that will be synced with Salesforce. CK wants to search and cross-reference records with the original source database.

What should a consultant recommend meeting this requirement?
A . Use the standard External ID field and map this to the source record ID value.
B . Use a custom External ID field and map this to the source record ID value.
C . Use the standard External ID field and map this to the Salesforce record ID value.
D . Use a custom field named External ID and map this to the Salesforce record ID value.

Answer: B