From an Ethical Viewpoint,the ABAB Design Is Preferred Over the ABA

From an ethical viewpoint,the ABAB design is preferred over the ABA design because

A) the ABAB design more powerfully rules out chance fluctuations.
B) the ABAB design is less expensive.
C) the ABAB design shows the reversibility of a treatment.
D) it does not seem right to end the design with the withdrawal of a treatment that may be beneficial to the participant.