Drill 22 · Reading & Writing · Hard Command of Evidence
SAT R&W Command of Evidence (Hard) — Drill 22 is a Reading & Writing practice drill covering Hard Command of Evidence. It contains 5 original questions created by Brian Stewart, a Barron's test prep author with over 20 years of tutoring experience.
Trees move water from soil to leaf against gravity, and these items probe how researchers separate competing explanations for a drought finding. Two of the five are data items -- one bar comparison, one table -- and the rest are finding items that ask which observation decides between rival mechanisms. As always, the trap distractors are true on their own terms but answer a slightly different question than the one posed.
In a mixed forest, researchers track growth in two tree species across a wet year and the dry year that followed. They want to know which species is more drought-sensitive: which one loses more growth when water runs short, not which simply grows more overall. For each species they record average ring width in the wet year and in the dry year and plot the four values as grouped bars.
Question 1. Which statement best identifies which species was more drought-sensitive, based on the graph?
Explanation: Correct: Drought sensitivity is the size of each species' wet-to-dry drop, and the ring-porous species' bars fall by more between the two years. That gap between the gaps is what the question asks for. A: A single-year height comparison is true from the bars but speaks to overall vigor in the wet year, not to which species drops more under drought. B: That both decline is true yet says nothing about which one declines more, which is the comparison at issue. C: Comparing the two species within the dry year alone is one reading off the bars, but drought sensitivity is the change across years, not the dry-year level.
During the dry year, the ring-porous trees stopped moving water up their trunks well before the diffuse-porous trees did. Researchers propose that the ring-porous trees' wide water-conducting vessels filled with air bubbles under drought stress -- a blockage called embolism -- physically cutting off the water column. A rival explanation is that these trees simply closed the tiny leaf pores, the stomata, to conserve water, so the flow stopped by choice rather than by damage. One account means the tissue was permanently damaged; the other means it was only idled and could recover.
Question 2. Which finding, if true, would most strongly support the embolism explanation over the stomata-closing explanation?
Explanation: Correct: Closed stomata reopen and flow resumes once water returns, but an air-blocked vessel stays blocked. Current-year vessels that remain air-filled and carry no water even after the soil rewets and the leaf pores reopen point to permanent embolism rather than a temporary stomatal shutdown. A: Early leaf drop can follow either embolism or water-saving stomatal closure, so it does not separate the two. B: A day-night swing in water use reflects evaporative demand and fits either explanation; it does not single out embolism. D: Equal wet-year transport describes normal conditions and says nothing about what stopped the flow during the drought.
A second question concerns why some individual trees of the drought-sensitive species kept growing through the dry year while their neighbors faltered. One researcher proposes that the survivors tapped a deeper, longer-lasting source of soil moisture than the others reached, and that rooting depth, not anything about the canopy, made the difference. To turn this into a test, she frames a prediction: if deep moisture access is the cause, then a specific pattern should appear when the trees are compared by where they got their water.
Question 3. Which result would best fit the prediction that follows from the deep-moisture explanation?
Explanation: Correct: The explanation is specifically about water depth, and tracer chemistry that ties continued growth to deep-soil water -- and faltering to dried shallow water -- is the direct test of that source. It confirms the predicted pattern. B: Crown width is a canopy trait, the very factor the explanation sets aside; it does not test water depth. C: An age difference offers a rival explanation rather than confirming the moisture-depth one. D: Shared loss of surface roots affects both groups alike and so cannot explain why only some kept growing.
Table: four study plots during the dry year
To find where the drought-sensitive species fared best, the team rates four plots on two measures during the dry year: depth to reliable soil moisture and canopy openness. In this species, ordinary roots reach down to about 120 cm, so reliable moisture counts as within reach only when it sits at 120 cm or shallower. And the canopy counts as closed enough to limit heat stress only when no more than about 30 percent of the sky is open above the trees. The team reasons the species should hold up only where reliable moisture is within reach AND the canopy is closed enough, so they look for the single plot that clears both thresholds.
Question 4. Based on the table, which plot meets both thresholds -- reliable moisture within root reach and a canopy closed enough to limit heat stress?
Explanation: Correct: Within reach means 120 cm or shallower; closed enough means 30 percent open or less. Plot X (90 cm, 22 percent) is the only plot that clears both thresholds at once. A: Plot W's moisture is within reach at 95 cm, but its canopy is 58 percent open, well above the 30 percent threshold, so it fails the heat-stress condition. C: Plot Y's canopy is closed enough at 25 percent open, but its moisture lies at 210 cm, beyond the 120 cm root reach, so it fails the moisture condition. D: Plot Z fails both thresholds: moisture at 240 cm is out of reach and the canopy is 60 percent open.
Reviewing the survivors, a researcher concludes that thicker bark caused the difference, noting that the trees that kept growing through the drought had thicker bark than those that faltered. She suggests the bark insulated the living trunk tissue and protected it from heat stress during the hottest weeks. A colleague is unconvinced. He points out that the thicker-barked trees were not otherwise comparable to the others: they differed in more than bark alone, and an unstated difference in where they grew, rather than the bark itself, could account for the pattern. Before crediting the bark, he says, one would have to rule out any background factor that the thick-barked and thin-barked trees did not share.
Question 5. Which finding, if true, would most weaken the claim that thicker bark caused the trees to keep growing through the drought?
Explanation: Correct: If the thick-barked trees also sat on deep soil that held more water while the otherwise similar thin-barked trees were on thin dry soil, then soil depth, not bark, could explain the survival -- a clean rival cause with the other factors held alike. That undercuts the bark claim. A: A height difference does not obviously help with drought and does not supply an alternative cause for surviving it. B: With the two groups matched for age, the age-bark link cannot explain the survival difference, so this is background rather than a competing cause. C: Equal wet-year growth removes a confound rather than introducing one, so it does not weaken the drought-year bark claim.