以下是CLASS(课程)表和INSTRUCTOR(教师)表中所存储的数据：
CLASS
|
| CLASS_ID | CLASS_NAME | HOURS_CREDIT | INSTRUCTOR_ID |
| 1 | Introduction to Accounting | 3 | 4 |
| 2 | Computer Basics | 3 | 1 |
| 3 | Tax Accounting Principles | 3 | 4 |
| 4 | American History | 3 | 2 |
| 5 | Basic Engineering | 3 | |
INSTRUCTOR
|
| INSTRUCTOR_ID | LAST_NAME | FIRST_NAME |
| 1 | Chao | Ling |
| 2 | Vanderbilt | Herbert |
| 3 | Wigley | Martha |
| 4 | Page | Albert |
要删除没有指派教师(INSTRUCTOR)的课程(CLASS)，在如下的DELETE语句中，哪一个将获取所希望的结果
A．DELETE class_id, class_name, hours_credit, instructor_id
FROM class
WHERE instructor_id IS NULL;
B．DELETE FROM class
WHERE instructor_id NOT IN
(SELECT instructor_id
FROM class);
C．DELETE FROM
instructor NATURAL JOIN class
WHERE instructor_id IS NOT NULL;
D．DELETE FROM class
WHERE instructor_id IS NULL;