The conventional process for tanning leather uses large amounts of calcium oxide and sodium sulfide. Tanning leather using biological catalysts costs about the same as using these conventional chemicals if the cost of waste disposal is left out of the comparison. However, nearly 20 percent less waste is produced with biological catalysts, and waste disposal is a substantial part of the overall cost of tanning. It is ttherefore less costly to tan leather if biological catalysts are used instead. Which one of the following is an assumption required by the argument? ['Leather tanned using the conventional process is not lower in quality than is leather tanned using biological catalysts.', 'Disposal of tanning waste produced with biological catalysts does not cost significantly more than disposal of the same amount of waste produced with the conventional process.', 'The biological catalysts that can be used in the tanning process are less costly by weight than are calcium oxide and sodium sulfide.', 'The labor costs associated with tanning leather using biological catalysts are not any greater than the labor costs associated with the conventional tanning process.']