问题：

[单选]

The EMP table contains these columns:
LAST_NAME VARCHAR2 (25)
SALARY NUMBER (6,2)
DEPARTMENT_ID NUMBER (6)
You need to display the employees who have not been assigned to any department. You write the SELECT statement:
SELECT LAST_NAME, SALARY, DEPARTMENT_ID
FROM EMP
WHERE DEPARTMENT_ID = NULL;
What is true about this SQL statement ?（）

A . The SQL statement displays the desired results.
B . The column in the WHERE clause should be changed to display the desired results.
C . The operator in the WHERE clause should be changed to display the desired results.
D . The WHERE clause should be changed to use an outer join to display the desired results.