United States. General Accounting Office. Environmental Protection Agency: Control of Air Pollution From New Motor Vehicles--Tier 2 Motor Vehicle Emissions Standards and Gasoline Sulfur Control Requirements, text, February 23, 2000; (https://digital.library.unt.edu/ark:/67531/metadc291086/m1/1/: accessed November 18, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; .