2.23 Fuel consumption for different types of vehicles. Refer to the previous two exercises. Those exercises examined data for vehicles that used regular fuel. Data are also available for vehicles that use several other types of fuel. There are 1067 vehicles in total. The variable Fuel has four different possible values: X, for regular fuel; Z, for premium fuel; D, for diesel; and E, for ethanol.

1. (a) Make a scatterplot of all of the data using different symbols or colors for the different fuel types.

2. (b) Does the relationship between CO₂ and highway fuel consumption depend upon the type of fuel that the vehicle uses? Explain your answer.