TOEFL Listening: ETS-TOEFL听力机经 - 2M24QDY1SI5NIR49P$

Why does the professor mention illumination? A. To explain why the researchers consider lighting variable to human work. B. To suggest a reason why the study was considered a failure. C. To compare the working conditions between the two manufacturing plants. D. To give an example of a working condition that was altered at a manufacturing factory.