Marley Got Back Her Student's Exams and Found That 42

Multiple Choice

Marley got back her student's exams and found that 42% of the questions were not answered (i.e., left blank) by over half of the students. Which of the following should she investigate:

A) that some of these questions were poorly written.
B) that there was not enough time to complete the test.
C) the content was not covered in class appropriately.
D) All of these.