When creating an object to model invoices, what should the Architect suggest, considering performance and data storage space?

Get Cloudy Consulting uses Salesforce for tracking opportunities (Opportunity) and currently has the following environment:

– An internal ERP system is in place for tracking services and invoicing.

– The ERP system supports SOAP API and CData for bi-directional integration between Salesforce and the ERP system.

– 950,000 opportunities exist; for each opportunity, the company sends one invoice per month during a 12month period.

Get Cloudy Consulting sales reps must view current invoice status and invoice amount from the opportunity page.

When creating an object to model invoices, what should the Architect suggest, considering performance and data storage space?
A . Create an external object Invoice_x with a Lookup relationship with Opportunity.
B . Create a custom object Invoice_c with a Lookup relationship with Opportunity.
C . Retrieve the current status from the ERP by using Streaming API, and display on the Opportunity page.
D . Create a custom object Invoice_c with a master-detail relationship with Opportunity.