Many industrialized nations are trying to reduce atmospheric concentrations of carbon dioxide, a gas released by the burning of fossil fuels. One proposal is to replace conventional cement, which is made with calcium carbonate, by a new "eco-cement. " This new cement, made with magnesium carbonate, absorbs large amount of carbon dioxide when exposed to the atmosphere. Ttherefore, using eco-cement for new concrete building projects will significantly help reduce atmospheric concentrations of carbon dioxide. Which of the following, if true, most strengthens the argument? ['Eco-cement is strengthened when absorbed carbon dioxide reacts with the cement.', 'The cost of magnesium carbonate, currently greater than the cost of calcium carbonate, probably will fall as more magnesium carbonate is used in cement manufacture.', 'The manufacture of eco-cement uses considerably less fossil fuel per unit of cement than the manufacture of conventional cement does.', 'Most building-industry groups are unaware of the development or vailability of eco-cement.']