以下是CLASS(课程)表和INSTRUCTOR(教师)表中所存储的数据： CL

问题：

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;