The emission of sulfur dioxide when high-sulfur coal is burned is restricted by law. New coal-burning plants usually comply with the law by installing expensive equipment to filter sulfur dioxide from their emissions. These new plants could save money by installing instead less expensive cleaning equipment that chemically removes most sulfur from coal before combustion. Which of the following, if known, would be most relevant to evaluating the claim above about how new coal-burning plants could save money? ['Whether the expense of installing the cleaning equipment in a new plant is less than the expense of installing the cleaning equipment in an older plant', 'Whether the process of cleaning the coal is more expensive than the process of filtering the emissions', 'Whether existing oil-burning plants are required to filter sulfur dioxide from their emissions', 'Whether existing plants that use the filtering equipment could replace this equipment with the cleaning equipment and still compete with new plants that install the cleaning equipment']