Examine the structure of the EMPLOYEES table:<br /> EMPLOYEE_ID NUMBER Primary Key<br /> FIRST_NAME VARCHAR2(25)<br /> LAST_NAME VARCHAR2(25)<br /> 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', ' ');