Examine the structure of the EMPLOYEES table: EMPLOYEE_ID NUMBER Primary Key FIRST_NAME VARCHAR2(25) LAST_NAME VARCHAR2(25) Which three statements insert a row into the table? （）

• A、INSERT INTO employees VALUES ( NULL, 'John', 'Smith');
• B、INSERT INTO employees( first_name, last_name) VALUES( 'John', 'Smith');
• C、INSERT INTO employees VALUES ( '1000', 'John', NULL);
• D、INSERT INTO employees (first_name, last_name, employee_id) VALUES ( 1000, 'John', 'Smith');
• E、INSERT INTO employees (employee_id) VALUES (1000);
• F、INSERT INTO employees (employee_id, first_name, last_name) VALUES ( 1000, 'John', ' ');