The article "Exhaust Emissions from Four-Stroke Lawn Mower Engines" (J. of the Air and Water Mgmnt. 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):

Construct scatter-plots of NOx emissions versus age. What appears to be the nature of the relationship between these two variables? The authors of the cited article commented on the relationship.

Engine Age Baseline Reformulated 1.88 5.93 5.54 2.67 6.53 0 0 27 1.72 4.38 4.06 1.26 5.31 Engine Age Baseline Reformulated 74 4.94 4.89 69 42 6 78910 16 9 014 57 3.37 344 74 1.24