Because dried peat moss, which is derived from sphagnum moss, contains no chemical additives and is a renewable resource, many gardeners use large amounts of it as a soil conditioner in the belief that the practice is environmentally sound. They are mistaken. The millions of acres of sphagnum moss in the world contribute more oxygen to the atmosphere than do all of the world' s rain forests combined, and the garden soil industry is depleting these areas much faster than they can renew themselves. Which one of the following principles, if valid, most helps to justify the argument's reasoning? ['A practice is environmentally sound if it helps to protect rain forests that contribute large amounts of oxygen to the atmosphere.', 'A practice is not environmentally sound if it significantly reduces the amount of oxygen entering the atmosphere.', 'Using a product may be environmentally unsound even if the product is a renewable resource and contains no chemical additive.', 'If the environmental benefits of a practice outweigh the environmental costs, that practice can be legitimately considered environmentally sound.']