An Instructor in Introductory Psychology Wanted to Know If the Textbook

An instructor in Introductory Psychology wanted to know if the textbook she was using was as effective as other textbooks on the market. In order to test this, she decided that she would teach the course in the fall with the text she typically uses, and in the spring she would use a new text. After doing this, she compared the students' final grades and students' responses to an 'end of semester' evaluation question. The question was "How effective was the text for this course?" She discovered that students' grades increased from a mean of 78 in the fall to 83 in the spring. She also discovered that the students' opinion of the course's text increased from a mean of 4.5 (on a 7-point scale, the higher the better) in the fall to 4.9 in the spring. After running the appropriate statistical test(s) , she discovered that both of these differences were statistically significant.
How might this study be improved?

A) randomly sample students for the course
B) randomly assign students to take the course in the Fall or Spring
C) have a different instructor repeat the experiment using the texts in the opposite order
D) all of the above