2. The article Exhaust Emissions from Four-Stroke LawnMower Engines (J. AirWater Manag. Assoc., 1997: 945—952) reported data from a study in which both a baseline gasoline mixture and a reformulated gasoline were used. Consider the following observations on age (yr) and NOx emissions (g/kWh):

Engine 1 2 3 4 5 Age 0 0 2 11 7 Baseline 1.72 4.38 4.06 1.26 5.31 Reformulated 1.88 5.93 5.54 2.67 6.53 Engine 6 7 8 9 10 Age 16 9 0 12 4 Baseline .57 3.37 3.44 .74 1.24 Reformulated .74 4.94 4.89 .69 1.42 Construct scatter plots of NOx emissions versus age.

What appears to be the nature of the relationship between these two variables? (Note: The authors of the cited article commented on the relationship.)