以下哪个特点不是国际多式联运所应具有的特点( )。
A．签订一个运输合同
B．采用一种运输方式
C．采用一次托运
D．一次付费
参考答案：B
解析：国际多式联运，是指根据一个多式联运合同，采用两种或两种以上的运输方式，由多式联运经营人把货物从一国境内运到另一国境内的行为。采用一种运输方式运输货物，通常情况下不认为是国际多式联运，所以本题应选择B。
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 |
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;