Which of the following best expresses the main idea of the passage ........

English ( Master) Reading Comprehension (C)

Read the passage below and answer the following questions.
In nearly all human populations a majority of individuals can taste the artificially synthesized chemical phenyl-thiocarbonide (PTC). However, the percentage varies dramatically--from as low as 60% in India to as high as 95% in Africa. That this polymorphism is observed in non-human primates as well indicates a long evolutionary history which, although obviously not acting on PTC, might reflect evolutionary selection for taste discrimination of other, more significant bitter substances, such as certain toxic plants.
A somewhat more puzzling human polymorphism is the genetic variability in earwax, or cerumen, which is observed in two varieties. Among European populations 90% of individuals have a sticky yellow variety rather than a dry, gray one, whereas in northern China these numbers are approximately the reverse. Perhaps like PTC variability, cerumen variability is an incidental expression of something more adaptively significant. Indeed, the observed relationship between cerumen and odorous bodily secretions, to which non-human primates and, to a lesser extent, humans pay attention, suggests that during the course of human evolution genes affecting body secretions, including cerumen, came under selective influence.

Which of the following best expresses the main idea of the passage?

Artificially synthesized chemicals might eventually serve to alter the course of evolution by desensitizing humans to certain tastes and odors.

Some human polymorphisms might be explained as vestigial evidence of evolutionary adaptations that still serve vital purposes in other primates.

Sensitivity to taste and to odors have been subject to far greater natural selectivity during the evolution of primates than previously thought.

Polymorphism among human populations varies considerably from region to region throughout the world.

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Explanation:

- The passage discusses the variability in human sensitivity to PTC and earwax types. It suggests that these polymorphisms might have roots in evolutionary adaptations shared with other primates.

- Option 1: The passage does not focus on artificially synthesized chemicals altering evolution; it looks at existing polymorphisms.

- Option 2: The passage suggests that these polymorphisms might be remnants of evolutionary traits important for other primates, like taste discrimination and body secretions. This aligns with the overall discussion.

- Option 3: While taste and odors are involved, the passage doesn't suggest these traits were more selective than previously thought.

- Option 4: It mentions variability but focuses on explaining this through evolutionary adaptation, not just regional differences.