Which of the following most logically completes the argument?

Which of the following most logically completes the argument?

Many advocate cost-benefit assessment of environmental policies. Although some costs and benefits are already in monetary terms, others, such as effects on the quality of

people's lives, are not. It has been proposed that these costs and benefits be assigned monetary values based on what people say they would pay to avoid or gain them.

However, while this appears to treat everyone's interests alike, it is biased against the interests of certain groups because