问题：

[多选]

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', ' ')