对于数据表：student(sno,sname,sex,age,grade)，查询学习成绩为空的学生的学号和姓名，下面语句正确的是______。
A．SELECT sno,sname FROM student WHERE grade=NULL
B．SELECT sno,sname FROM student WHERE grade IS NULL
C．SELECT sno,age FROM student WHERE grade IS NULL
D．SELECT sno,age FROM student WHERE grade=NULL