Question 196·200 Super-Hard SAT Reading Questions·Information and Ideas
To estimate when a bare rock surface was exposed, geologists sometimes use lichenometry, which relates the size of lichens growing on the surface to its exposure age. Geologist Amara Okafor and her colleagues measured the five largest lichen patches on independently dated rock surfaces ranging from 10 to 180 years old in two nearby ravines. Within each ravine, the median patch diameter increased with surface age through approximately 70 years but varied little among older surfaces. Furthermore, on surfaces of the same age, patches in the shaded, humid ravine were consistently larger than those in the exposed, drier ravine. The researchers therefore concluded that ______
Which choice most logically completes the text?
For an inference question, identify each major finding and require the answer to accommodate all of them. Be cautious of choices that correctly reflect one finding but ignore another; here, a valid conclusion must address both the age-related plateau and the environmental difference between ravines.
Hints
Separate younger and older surfaces
Consider whether lichen diameter changes equally across the entire 10-to-180-year range.
Compare the ravines
Focus on what the different lichen sizes on same-aged surfaces reveal about environmental effects.
Combine both findings
Look for a conclusion that recognizes both the method's useful range and the conditions under which comparisons are valid.
Step-by-step Explanation
Identify the age-related pattern
Within each ravine, lichen diameter increased with exposure age only through about 70 years. Because diameters varied little after that point, size could not reliably distinguish among the older surfaces.
Account for habitat conditions
Same-aged surfaces had different lichen sizes depending on whether the ravine was humid and shaded or dry and exposed. Therefore, a size measured in one habitat cannot be interpreted using observations from another habitat without additional evidence.
Choose the supported conclusion
The conclusion must reflect both limitations while preserving the method's usefulness for younger, environmentally comparable surfaces. Therefore, the correct answer is lichen size may help order the exposure ages of younger surfaces within comparable habitats, whereas older surfaces or surfaces from different habitats require evidence beyond size alone.