Explanation
Correct answer: C.
Choice C is the best answer because it most logically completes the text’s discussion of Vámos and colleagues’ experiment with four duckweed ecotypes. According to the text, the researchers hypothesized that each ecotype is adapted to its local habitat in ways that help it grow more effectively and better tolerate pollutants. To test this, they grew each ecotype in water collected from all four habitats and exposed them to three different zinc levels. If the hypothesis were correct, each ecotype would be expected to thrive more in water from its own habitat than in the other waters and to respond to zinc levels differently depending on the level of zinc pollution in its home environment. However, the results showed that every ecotype grew just as well in every water sample, and that zinc boosted growth the same way across all ecotypes no matter the concentration. In other words, the experiment found no evidence that these ecotypes have specifically adapted to their local water or their local zinc exposure. It follows, then, that if each ecotype is actually locally adapted, it is adapted not to the water conditions in its habitat but to other environmental factors—such as local light or temperature levels, which the text notes the researchers didn’t account for in their experiment.
Why the other choices are wrong
Choice A
Choice A is incorrect because the findings presented in the text suggest only that the ecotypes’ genetic and geographic differences don’t represent adaptations to local water conditions and zinc levels, not that the differences don’t represent adaptations to any local environmental conditions. Indeed, the text notes that the researchers didn’t account for light or temperature levels in each habitat, suggesting the possibility that the ecotypes are adapted to those or other local environmental factors.
Choice B
Choice B is incorrect because it contradicts the findings presented in the text. The text states that zinc consistently enhanced growth regardless of concentration, meaning the ecotypes all responded similarly to zinc. This uniform response indicates there aren’t significant differences in their resistance to zinc pollution.
Choice D
Choice D is incorrect because the findings presented in the text don’t demonstrate that the researchers’ hypothesis of local adaptation isn’t supported. That the ecotypes grew equally well in all of the water samples and equally benefited from added zinc at all levels, regardless of zinc levels in their natural environments, suggests only that the ecotypes aren’t adapted to their local water conditions and zinc levels; these findings don’t rule out the possibility that the ecotypes are adapted to other local factors in their habitats, such as light or temperature (which the text notes the researchers didn’t examine).