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单项选择题 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';

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