Examine the structure of the EMPLOYEES and NEW_EMPLOYEES tables:
EMPLOYEES
EMPLOYEE_ID NUMBER Primary Key
FIRST_NAME VARCHAR2(25)
LAST_NAME VARCHAR2(25)
HIRE_DATE DATE
NEW_EMPLOYEES
EMPLOYEE_ID NUMBER Primary Key
NAME VARCHAR2(60)
Which DELETE statement is valid?（）

A.SELECT ADD_MONTHS(MAX(hire_Date), 6) FROM EMP;
B.SELECT ROUND(hire_date) FROM EMP;
C.SELECT sysdate-hire_date FROM EMP;
D.SELECT TO_NUMBER(hire_date + 7) FROM EMP;

A.DELETE employees;
B.DESCRIBE employees;
C.ROLLBACK TO SAVEPOINT C;
D.GRANT SELECT ON employees TO SCOTT;
E.ALTER TABLE employees SET UNUSED COLUMN sal;
F.SELECT MAX(sal) FROM employees WHERE department_id = 20;

A.The UNIQUE constraint does not permit a null value for the column.
B.A UNIQUE index gets created for columns with PRIMARY KEY and UNIQUE constraints.
C.The PRIMARY KEY and FOREIGN KEY constraints create a UNIQUE index.
D.The NOT NULL constraint ensures that null values are not permitted for the column.