What is the optimal way to implement this?

A company represents their customers as Accounts in Salesforce. All customers have a unique Customer_Number__c that is unique across all of the company’s systems. They also have a custom Invoice__c object, with a Lookup to Account, to represent invoices that are sent out from their external system. This company wants to integrate invoice data back into Salesforce so Sales Reps can see when a customer is paying their bills on time.

What is the optimal way to implement this?
A . Use Salesforce Connect and external data objects to seamlessly import the invoice data into Salesforce without custom code.
B. Ensure Customer_Number__c is an External ID and that a custom field Invoice_Number__c is an External ID and Upsert invoice data nightly.
C. Create a cross-reference table in the custom invoicing system with the Salesforce Account ID of each Customer and insert invoice data nightly.
D. Query the Account Object upon each call to insert invoice data to fetch the Salesforce ID corresponding to the Customer Number on the invoice

Answer: B