单项选择题
设有dep和student表，数据如下：
| dep | |
| dno | dname |
| 10 | 数学 |
| 20 | 外语 |
| 30 | 计算机 |
| 40 | 电子工程 |
| 50 | 自动化 |
| student | ||
| sno | sname | dno |
| 980001 | 李宏 | 10 |
| 980002 | 王明 | 10 |
| 980012 | 刘华 | 50 |
| 980015 | 朱敏 | 50 |
现有如下所示的查询结果：
| dname |
| 外语 |
| 计算机 |
| 电子工程 |
下列查询语句中，能够得到此查询结果的是（）。
A．select dname from dep right join student as s on dep. dno= s. dno where sno is null
B．select dname from dep right join student as s on dep. dno=s, dno where dep. dno is null
C．select dname from dep left join student as s on dep. dno=s, dno where sno is null
D．select dname from dep left join student as s on dep. dno=s, dno where dep. dno is null