To determine which pair of quantitative variables would likely produce a straight line on a scatterplot, we need to consider whether there is a direct linear relationship between the two variables.
Among the options provided:
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Time spent playing video games and adult shoe size: There is unlikely to be a linear relationship between these two variables, as they are not inherently connected.
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Time spent studying for an exam and the grade on the exam: There is typically a positive linear relationship here. Generally, more time spent studying correlates with higher grades, so this could produce a straight line on a scatterplot.
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Time it takes to get to the grocery store and the number of grocery items purchased: This relationship is more complex. The time it takes to get to the grocery store does not directly correlate with the number of items purchased; it could vary based on how full a person's grocery list is or other factors.
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Time spent mowing the lawn and the number of snakes in the yard: There's no expected linear relationship here either, as the presence of snakes is likely independent of the time spent mowing the lawn.
Based on this analysis, the pair that is most likely to show a straight line on a scatterplot is:
Time spent studying for an exam and the grade on the exam.