Answer:
Option C. Use an instrument that measures student understanding.
Explanation:
Thinking process:
The basis of the experiment is to measure student's understanding. Therefore, this is the primary focus of the experiment. In the test, there is an observation of the students by the professor. At the moment, there is no quantitative analysis to measure the student's understanding from the CD distribution. Thus, another method is needed to ensure the validity of the results. In this case, an instrument that measures student's understanding is needed.