How does Magento store comments regarding different entities (invoice, credit memo, shipment)?

A.
Each comment is a separate entity with a type attribute which identifies the owner of the comment. All
comments are stored in one sales_comment table.

B.
Comments to invoice, credit memo and shipment are different entities stored in the different tables.

C.
All comments belong directly to the order, and are stored in the sales_flat_order_comments table.

D.
Each comment is an attribute of the appropriate entity (invoice, credit memo, shipment).