Based on the data inserted to university database, one of the following statements is a valid insert statement into exam_results table:

a.

Insert into exam_results values(2018101,8446,first exam);

b.

Insert into exam_results values(2018101,8446,null,null,null);

c.

Insert into exam_results values(2017101,9442,first exam,20,20);

d.

Insert into exam_results values(2017199,9442,second exam,20,20);