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