BillingService needs to send audit records through interchangeable destinations. Which re…

BillingService needs to send audit records through interchangeable destinations. Which relationship keeps the boundary smallest?

1. BillingService inherits FileAuditSink.
2. BillingService accepts an AuditSink interface in its constructor.
3. BillingService writes to a companion-object list.
4. Every billing method creates its own FileAuditSink.