问题：

[单选]

Examine the description of the EMPLOYEES table:
EMP_ID NUMBER(4) NOT NULL
LAST_NAME VARCHAR2(30) NOT NULL
FIRST_NAME VARCHAR2(30)
DEPT_ID NUMBER(2)
Which statement produces the number of different departments that have employees with last name Smith?（）

A . SELECT COUNT(*) FROM employees WHERE last_name='Smith';
B . SELECT COUNT(dept_id) FROM employees WHERE last_name='Smith';
C . SELECT DISTINCT(COUNT(dept_id)) FROM employees WHERE last_name='Smith';
D . SELECT COUNT(DISTINCT dept_id) FROM employees WHERE last_name='Smith';
E . SELECT UNIQUE(dept_id) FROM employees WHERE last_name='Smith'