A constraint in a table is defined with the INITIALLY IMMEDIATE clause. You executed the ALTER TABLE command with the ENABLE VALIDATE option to enable the constraint that was disabled. What are the two effects of this command？（）

• A、It fails if any existing row violates the constraint.
• B、It does not validate the existing data in the table.
• C、It enables the constraint to be enforced at the end of each transaction.
• D、It prevents insert， update， and delete operations on the table while the constraint is in the process of being enabled.