单选题

请问如下的sql语句正确的是

Question

mysql数据库有选课表learn(student_id int,course_id int),字段分别表示学号和课程编号，现在想获取每个学生所选课程的个数信息，请问如下的sql语句正确的是

A. select student_id,sum(course_id)from learn

B. select student_id,count(course_id)from learn group by student_id

C. select student_id,count(course_id)from learn

D. select student_id,sum(course_id)from learn group by student_id