某客户开仓买入大豆期货合约20手（1手=10吨），成交价格为2020元／吨，当天平仓10手合约，成交价格为2040元／吨，当日结算价格为2010元／吨，交易保证金比例为5%，则该客户当日平仓盈亏是（）元，持仓盈亏是（）元。
A.-2000；1000
B.-1000；2000
C.1000；-2000
D.2000；-1000
参考答案：D
解析：当日平仓盈亏=（2040-2020）×10×10=2000（元）；当日持仓盈亏=（2010-2020）×10×10=-1000（元）。
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;