How can this requirement be met?

Universal Containers is importing 10,000 contact records in Salesforce. They want to avoid duplicate records from being created during the import.

How can this requirement be met?
A . When importing the file, select the "Prevent Duplicates" option on the last step of the Data Loader.
B . Include in a column in the import file that corresponds to a field in Salesforce that requires unique values.
C . After importing all the contacts, run a duplicate check report, export the report to a CSV file, and run a mass delete to purge any duplicates.
D . After importing all of the contact records, review all records created and manually merge or delete any duplicate records.

Answer: B