SAT Reading and Writing · Command of Evidence: Quantitative Practice Question

A study measured air-quality index (AQI; lower is cleaner) in four neighborhoods that varied by traffic level (low/high) and tree cover (low/high): Block 1, low traffic + low trees, AQI 60; Block 2, low traffic + high trees, AQI 45; Block 3, high traffic + low trees, AQI 110; Block 4, high traffic + high trees, AQI 85. The researchers concluded that <em> both </em> tree cover and traffic level independently influence AQI. The data most directly supporting the <em> independent </em> contribution of tree cover is ______

Which choice most logically completes the text using the data above?

A
$Block 2's low AQI of 45.$
B
$Block 3's high AQI of 110.$
C
$the comparison of Block 1 (60) vs. Block 2 (45)—same low traffic, but high tree cover lowers AQI by 15.$
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D
$the comparison of Block 1 vs. Block 4.$

Explanation

To show trees independently matter, hold traffic constant. Block 1 vs. Block 2 does this (both low traffic), with high tree cover reducing AQI from 60 to 45. Choice C uniquely isolates the tree variable.

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