Dr Smitherman Conducted a Study 5 Years Ago, and His

Dr. Smitherman conducted a study 5 years ago, and his graduate student now recommends that they conduct the study again to see if the effect still occurs. Dr. Smitherman says, "No, I cannot do that study now; I think it is unethical." Which of the following is NOT a reasonable explanation for Dr. Smitherman's response?

A) There were no ethical guidelines 5 years ago, but there are now.
B) He might have changed his thinking due to a bad experience with some of the participants from the original study.
C) Public opinion about that type of research has changed.
D) New findings have made such research less beneficial.