Many environmentalists have urged environmental awareness on consumers, saying that if we accept moral responsibility for our effects on the environment, then products that directly or indirectly harm the environment ought to be avoided. Unfortunately it is usually impossible for consumers to assess the environmental impact of a product, and thus impossible for them to consciously restrict their purchases to environmentally benign products. Because of this impossibility there can be no moral duty to choose products in the way these environmentalists urge, since __. Which one of the following principles provides the most appropriate completion for the argument? ['the morally right action is always the one whose effects produce the least total harm', 'a moral duty to perform an action is never based solely on the effects the action will have on other people', 'a person cannot possibly have a moral duty to do what he or she is unable to do', 'where a moral duty exists, it supersedes any legal duty and any other kind of duty']