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.

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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