How should the app builder configure the InvoiceNumber field?

Universal Containers has a new custom object for Invoices that includes an InvoiceNumber field, Before the Invoice object can be used, invoices will be migrated from an external system maintaining their current InvoiceNumber. After the migration, salesforce will be the system of record and each new Invoice created in Salesforce must have a unique InvoiceNumber.

How should the app builder configure the InvoiceNumber field?
A . Create a Text filed for the original InvoiceNumber and an AutoNumber field for the Salesforce InvoiceNumber.
B . Create a Text field and mark it as a unique external ID field.
C . Create a Text field, then change it to AutoNumber after the migration.
D . Create an AutoNumber field and migrate the Invoices

Answer: B