Chemical fertilizers not only create potential health hazards, they also destroy earthworms, which are highly beneficial to soil. For this reason alone the use of chemical fertilizers should be avoided. The castings earthworms leave behind are much richer than the soil they ingest, thus making a garden rich in earthworms much more fertile than a garden without them. Which one of the following most accurately expresses the main conclusion of the argument? ['The castings that earthworms leave behind are much richer than the soil they ingest.', 'Chemical fertilizers destroy earthworms.', 'The use of chemical fertilizers should be avoided.', 'Earthworms are highly beneficial to soil.']