SAT R&W Command of Evidence (Hard) — Drill 30

Question 1. Based on the table, which depth meets both conditions the cold-water fish needs?

• A) 2 meters
• B) 6 meters
• C) 10 meters
• D) 8 meters ✓

Explanation: Correct: The 8-meter depth is the only one to satisfy both conditions: its dissolved oxygen is 6.2 milligrams per liter (at least 5) and its temperature is 13 degrees Celsius (no warmer than 15). Both conditions hold at once. A: At 2 meters the oxygen is ample at 8.5 milligrams per liter, but the water is 21 degrees Celsius, well above the 15-degree limit, so it fails the temperature condition. B: At 6 meters the temperature is a suitable 14 degrees Celsius, but the oxygen has fallen to 4.1 milligrams per liter, below the 5-milligram floor, so it fails the oxygen condition. C: At 10 meters the water is cold at 9 degrees Celsius but its oxygen is only 1.5 milligrams per liter, far below the floor, so it fails on oxygen.

Question 2. Which finding, if true, would most strongly support the cooling account over the wind account?

• A) Turnover each autumn is reliably followed within a week or two by a noticeable surge of algae growth across the lake's sunlit surface waters.
• B) In a year when autumn winds stayed unusually weak, turnover still arrived on schedule once the surface water had chilled to match the depths. ✓
• C) The lake sits in a wide, open basin with low surrounding ground, leaving its surface exposed to wind sweeping across it from nearly every direction.
• D) The lake's surface water reaches its warmest point in late summer, a few weeks before the autumn turnover begins to break the layering down.

Explanation: Correct: Turnover occurred once cooling erased the density difference between surface and depths, even without unusually strong winds, so the result favors cooling as the trigger over strengthened winds as the main cause. Wind still supplies mixing energy in a real lake, but here it was the loss of the density barrier, not a burst of wind, that let the lake overturn on schedule. A: An algae surge after turnover is a consequence of mixing, not a clue to whether wind or cooling caused it. C: An open, wind-exposed basin would if anything support the wind account; it does not favor cooling. D: That the surface is warmest weeks before turnover sets the seasonal scene but does not separate cooling from wind as the trigger.

Question 3. Which passage most directly supports the student's claim about the middle transition layer acting as a barrier to mixing?

• A) "The surface layer warms first as spring advances, and by the height of midsummer it stands several degrees warmer than the still water lying far below it, the two no longer trading heat freely across the depth of the lake."
• B) "Oxygen produced by plants near the sunlit surface is steadily consumed by the slow decay of organic matter settling into the sediments at the bottom of the deeper lakes, which can leave those depths starved of it by late summer."
• C) "Across the narrow mid-depth band, water just above and just below differs enough in density that turbulence crossing the band is strongly damped, so the upper and lower layers remain largely separate until fall." ✓
• D) "Wind blowing across a broad lake can push surface water toward the downwind shore, tilting the warm layer."

Explanation: Correct: The claim is that the steep mid-depth band blocks mixing between the upper and lower water. The keyed passage captures that without naming it outright: a density difference across the narrow band damps the turbulence that would cross it, so the layers stay separate until fall -- which is what acting as a barrier to mixing means. A: This explains why the surface ends up warmer than the depths, setting up the layering but not stating that the middle layer blocks mixing. B: This is about where oxygen is made and used up, a separate point that says nothing about a barrier to vertical mixing. D: Wind tilting the warm layer describes a surface disturbance, not the transition layer resisting mixing between top and bottom.

Question 4. Which result would best fit the prediction that follows from the denser-inflow proposal?

• A) Dye added to the inflow disappears from the surface near the stream mouth and reappears in samples drawn from near the lake bottom. ✓
• B) Dye added to the incoming stream spreads in a thin film across the warm surface layer and lingers there for days.
• C) Dye added to the incoming stream stays plainly detectable in the moving stream water itself right up to the point where the stream finally meets the open lake.
• D) Dye added to the incoming stream gradually fades from view over time as it is steadily diluted into the much larger surrounding volume of the whole lake.

Explanation: Correct: The proposal predicts the dense inflow dives beneath the surface and runs deep. Dye that vanishes from the surface at the stream mouth and shows up in bottom samples traces exactly that plunging path, fitting the prediction. B: Dye filming across the warm surface and staying is the opposite of the prediction; it is what a buoyant, less dense inflow would do. C: Dye still in the stream before it reaches the lake only shows the tracer was present; it says nothing about where the inflow goes once inside. D: Dye fading by dilution could happen at any depth and tells you the lake is large, not whether the inflow sank or spread.

Question 5. Which statement best identifies where the profile marks the transition layer?

• A) Temperature on the profile is highest right at the surface and lowest at the greatest depth the probe reached.
• B) Temperature changes only a little from reading to reading as the probe passes down through the warm, well-mixed water near the lake's surface.
• C) Over a narrow band at middle depth the temperature drops far more steeply with depth than it does above or below that band. ✓
• D) Temperature changes only a little from reading to reading as the probe passes down through the cold, near-uniform water close to the lake's bottom.

Explanation: Correct: The transition layer is the band where temperature changes most over a short interval of depth. Because temperature is on the horizontal axis and depth on the vertical one, that band shows up as the middle stretch where the curve swings sharply toward the cold (left) side as the probe descends just a few meters. The more upright stretches near the surface and the bottom are layers where temperature barely changes with depth. A: Warmest at top and coldest at the bottom is true of the whole profile but only states the end points; it does not locate the band of steepest change. B: Little change through the surface water describes the warm layer above the transition, not the steep band itself. D: Little change through the bottom water describes the cold layer below the transition, again not the steep band that defines it.