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', ' ')