Rivers rich in sediment appear yellow, while increases in red algae make rivers appear red. To track things like the...
GMAT Information and Ideas : (Ideas) Questions
Rivers rich in sediment appear yellow, while increases in red algae make rivers appear red. To track things like the sediment or algae content of large US rivers, John R. Gardner and colleagues used satellite data to determine the dominant visible wavelengths of light measured for various segments of these rivers. The researchers classified wavelengths of \(495\text{ nanometers (nm)}\) and below as red, wavelengths between \(495\text{ nm}\) and \(560\text{ nm}\) as blue, and wavelengths of \(560\text{ nm}\) and above as yellow. The researchers concluded that for the Missouri River, segments flowing into lakes tend to carry more sediment than those flowing out of lakes.
Which finding, if true, would most directly support the researchers' conclusion?
The segments of the Missouri River that had higher levels of chlorophyll-a, which contributes to the green color of photosynthetic organisms, have dominant wavelengths of light between 490 and 560 nm.
In lakes through which segments of the Missouri River pass, the dominant wavelength of light tended to be above 560 nm near the lakes' shores and below 560 nm in the lakes' centers.
The majority of the segments of the Missouri River were found to have dominant wavelengths of light significantly higher than 560 nm.
Segments of the Missouri River flowing into lakes typically had dominant wavelengths of light above 560 nm, while segments flowing out of lakes typically had dominant wavelengths below 560 nm.
Step 1: Decode and Map the Passage
Part A: Create Passage Analysis Table
| Text from Passage | Analysis |
|---|---|
| "Rivers rich in sediment appear yellow, while increases in red algae make rivers appear red." |
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| "To track things like the sediment or algae content of large US rivers, John R. Gardner and colleagues used satellite data to determine the dominant visible wavelengths of light measured for various segments of these rivers." |
|
| "The researchers classified wavelengths of 495 nanometers (nm) and below as red, wavelengths between 495 and 560 nm as blue, and wavelengths of 560 nm and above as yellow." |
|
| "The researchers concluded that for the Missouri River, segments flowing into lakes tend to carry more sediment than those flowing out of lakes." |
|
Step 2: Interpret the Question Precisely
What's being asked? Which finding would provide the strongest evidence to support the researchers' conclusion about Missouri River sediment patterns.
What type of answer do we need? Supporting evidence that backs up the claim that segments flowing INTO lakes carry more sediment than segments flowing OUT.
Any limiting keywords? Strongest evidence
Step 3: Prethink the Answer
- We know sediment makes rivers appear yellow (from the opening)
- We know yellow corresponds to wavelengths of 560 nm and above
- The conclusion states segments flowing INTO lakes have more sediment than those flowing OUT
- Therefore, segments flowing INTO lakes should show higher wavelengths (\(\geq 560 \text{ nm}\) = yellow)
- And segments flowing OUT should show lower wavelengths (\(\lt 560 \text{ nm}\) = red or blue)
The segments of the Missouri River that had higher levels of chlorophyll-a, which contributes to the green color of photosynthetic organisms, have dominant wavelengths of light between 490 and 560 nm.
✗ Incorrect
- Discusses chlorophyll-a and green color, not sediment content
- Doesn't compare segments flowing into vs. out of lakes
In lakes through which segments of the Missouri River pass, the dominant wavelength of light tended to be above 560 nm near the lakes' shores and below 560 nm in the lakes' centers.
✗ Incorrect
- Describes wavelength patterns within lakes themselves
- Doesn't address river segments flowing into or out of lakes
The majority of the segments of the Missouri River were found to have dominant wavelengths of light significantly higher than 560 nm.
✗ Incorrect
- States that most Missouri River segments had wavelengths above \(560\text{ nm}\)
- Doesn't differentiate between segments flowing in vs. out of lakes
Segments of the Missouri River flowing into lakes typically had dominant wavelengths of light above 560 nm, while segments flowing out of lakes typically had dominant wavelengths below 560 nm.
✓ Correct
- Shows segments flowing INTO lakes had wavelengths above \(560\text{ nm}\) (yellow = high sediment)
- Shows segments flowing OUT of lakes had wavelengths below \(560\text{ nm}\) (red/blue = low sediment)
- Perfectly matches our prediction