The passage starts off with a pretty simple statement. If we accept moral responsibility (AMR) for our effects on the environment, then we should avoid harmful products (AHP).
AMR - - - - - - - - - - > AHP
Then, a complication is added. Consumers do not know which products are harmful. Therefor, consumers are not able to avoid these products.
The author concludes that, because consumers can't avoid harmful products, they are relieved of their moral obligation to do so. Why? We have to choose a principle that reinforces this conclusion.
Answer choice B completes the argument perfectly. A person cannot have a moral duty to do what he/she is unable to do. This basically repeats the argument about harmful products, but it does so in more general terms. When it comes to principle questions, the challenge is applying general rules to the more specific scenario given in the passage.
