Earthworms, vital to the health of soil, prefer soil that is approximately neutral on the acid-to-alkaline scale. Since decomposition of dead plants makes the top layer of soil highly acidic, application of crushed limestone, which is highly alkaline, to the soil' s surface should make the soil more attractive to earthworms. Which one of the following is an assumption on which the argument depends? ['As far as soil health is concerned, aiding the decomposition of dead plants is the most important function performed by earthworms.', 'By itself, acidity of soil does nothing to hasten decomposition of dead plants.', 'Crushed limestone contains available calcium and magnesium, both of which are just as vital as earthworms to healthy soil.', "After its application to the soil's surface, crushed limestone stays in the soil's top layer long enough to neutralize some of the top layer's acidity."]