Which of the following explanations of schemas in a data base system is correct?
A. “Schema” is a general term for data base operations, including data insertion, updating, deleting, and retrieval. B. A schema is a virtual table from the viewpoint of the user, not an actual table. C. . “Schema” is a general term for various restrictions and conditions that are needed in order to keep a data base in a perfect condition. D. A schema is a collection of data definitions, including data characteristics, formats, and relationships with other data.