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Craft and Structure / Text Structure and Purpose Difficulty: Hard

Dubautia carrii is a species in a family of plants known collectively as the silversword alliance, all of which grow only on the Hawaiian Islands. Members of this alliance exhibit an extraordinary range of phenotypes, with some species maturing into vines and others into shrubs and trees. All species in the alliance descended from a single ancestral tarweed plant that arrived on the islands around 5 million years ago. The tarweed’s descendants diversified into distinct species as they adapted to live in the wide variety of habitats found on the Hawaiian Islands.

Which choice best states the main purpose of the text?

To indicate the specific tarweed ancestor of all plants that grow on the Hawaiian Islands and explain why the plants have such varied physical characteristics

To describe the specific habitat where Dubautia carrii are found and identify other plants that share a common ancestor with them

To describe the silversword alliance and explain how the plant family became so varied

To advance the claim that all plants on the Hawaiian Islands are part of the silversword alliance and list possible ancestors of the plants that make up the alliance

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Explanation

Choice C is the best answer because it most accurately describes the main purpose of the text, which is to describe the family of plants known as the silversword alliance and explain why the plants came to exhibit "an extraordinary range of phenotypes." The text introduces the silversword alliance as a family of plants unique to the Hawaiian Islands, presenting those that mature into vines and those that mature into shrubs and trees as examples of different phenotypes. The text concludes by explaining that these plants are all descendants of "a single ancestral tarweed plant" that arrived 5 million years ago in the Hawaiian Islands, where a "wide variety of habitats" drove the plant’s descendants to adapt and thus differentiate themselves, diversifying into a group of related but distinct species with different physical characteristics.

Choice A is incorrect. Although the text explains why the plants within the silversword alliance have varied physical characteristics, the text doesn’t suggest that tarweed is the ancestor of all plants on the Hawaiian Islands. The text states instead that all members of the silversword alliance descend from a single tarweed ancestor. Choice B is incorrect because while Dubautia carrii is mentioned as an example of a species in the silversword alliance, no details about its particular habitat are provided. And although the text indicates that the Hawaiian Islands are home to "a wide variety of habitats," it doesn’t describe the specific habitat where Dubautia carrii is found. The text discusses the silversword alliance as a whole rather than focusing on Dubautia carrii or any one species within this family. Choice D is incorrect. Although the silversword alliance is only found on the Hawaiian Islands and includes "an extraordinary range of phenotypes," the text doesn’t claim that all plants on the Hawaiian Islands are part of the alliance. Furthermore, the text indicates that these plants all "descended from a single ancestral tarweed plant," not that there are multiple possible candidates for that ancestor plant.