What common error should the data architect be aware of when using upsert?

To avoid creating duplicate Contacts, a customer frequently uses Data Loader to upsert Contact records into Salesforce.

What common error should the data architect be aware of when using upsert?
A . Errors with duplicate external Id values within the same CSV file.
B . Errors with records being updated and inserted in the same CSV file.
C . Errors when a duplicate Contact name is found cause upsert to fail.
D . Errors with using the wrong external Id will cause the load to fail.

Answer: A