SAT Question 973632d2: Rhetorical Synthesis answer and explanation

English - Rhetorical Synthesis - Present Information
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While researching a topic, a student has taken the following notes:

• In North America, woodlands have expanded into areas that were once grasslands. • Thomas Rogers and F. Leland Russell of Wichita State University investigated whether woodland expansion is related to changes in climate. • Rogers and Russell analyzed core samples from oak trees on a site that was not wooded in the past and indexed the age of the trees with historical climate data to see if tree populations and climate were correlated. • Tree population growth was associated with dry intervals. • Droughts may have played a role in woodland expansion.
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Difficulty:Hard
The student wants to emphasize the aim of the research study. Which choice most effectively uses relevant information from the notes to accomplish this goal?