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 . 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.
B . Query the Account Object upon each call to insert invoice data to fetch the Salesforce ID corresponding to the Customer Number on the invoice.
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 . Use Salesforce Connect and external data objects to seamlessly import the invoice data into Salesforce without custom code.

Answer: A