Question: Groundwater contamination in wells. In New Hampshire, about half the counties mandate the use of reformulated gasoline. This has lead to an increase in the contamination of groundwater with methyl tert-butyl ether (MTBE). Environmental Science and Technology (January 2005) reported on the factors related to MTBE contamination in public and private New Hampshire wells. Data were collected for a sample of 223 wells and saved in the MTBE file. (Selected observations are listed in the table at the bottom of p. 343.) The list of potential predictors of MTBE level (micrograms per liter) include well class (public or private), aquifer (bedrock or unconsolidated), pH level (standard units), well depth (meters), amount of dissolved oxygen (milligrams per liter), distance from well to nearest fuel source (meters), and percentage of adjacent land allocated to industry. Apply a variable screening method to the data to find a small subset of the independent variables that are the best predictors of MTBE level.