叶蓝绿色或蓝黑色，偶可见膜质托叶鞘的药材是
A．侧柏叶
B．蓼大青叶
C．大青叶
D．枇杷叶
E．番泻叶
参考答案：B
已知学生选课信息表：sc(sno，cno，grade)。查询“至少选修了一门课程，但没有学习成绩的学生学号和课程号”的SQL语句是（ ）。
A．SELECT sno，cno FROM sc WHERE grade=NULL
B．SELECT sno，cno FROM sc WHERE grade IS‘’
C．SELECT sno，cno FROM sc WHERE grade IS NULL
D．SELECT sno，cno FROM sc WHERE grade=‘’
-Is this your pencil sharpener?
-__________. It is Jim's pencil sharpener.[ ]
A. Yes, I am
B. Yes, it is
C. No, it isn't
D. No, it is
