Southwest Airlines is able to keep fares low, in part because of relatively low maintenance costs on its airplanes. One of the main reasons for the low maintenance costs is that Southwest flies only one type of aircraft: the Boeing 737. However, Southwest flies three different versions of the 737. Suppose Southwest decides to conduct a study to determine whether there is a significant difference in the average annual maintenance costs for the three types of 737s used. a. State a measurable dependent variable for such a study. b. State a factor that might affect the independent variable, and list at least three levels.