Evaluate the set of SQL statements: What is true about the set?（）

• A、The DESCRIBE DEPT statement displays the structure of the DEPT table.
• B、The ROLLBACK statement frees the storage space occupies by the DEPT table.
• C、The DESCRIBE DEPT statement returns an error ORA-04043: object DEPT does not exist.
• D、The DESCRIBE DEPT statement displays the structure of the DEPT table only if there is a COMMIT statement introduced before the ROLLBACK statement.