The question asks, "Which one of the following most accurately describes the author’s purpose in the last paragraph of the passage?"
The last paragraph gives us an implication of the new evidence discussed earlier in the passage, saying that it provides evidence to overturn the assumption that parasitic interactions evolve over time to symbiosis. The researchers determined that this assumption didn't apply to fungi, as they found fungi that are benign and harmful in the early and late stages of fungus evolutionary history. This tells us that the author believes that fungi can begin being benign and symbiotic but then later on become harmful to their hosts.
We're looking for the answer choice that most closely aligns with our summary of the last paragraph.
You asked about (A).
(A) says, "to suggest that new research overturns the assumption that lichen-forming fungi are primarily symbiotic, rather than parasitic, organisms."
The problem with this answer choice is that the research did not overturn the assumption that lichen-forming fungi are primarily symbiotic, rather than parasitic, organisms. From the information we have, the research does not change the fact that a lichen consists of a fungus living in symbiosis with an alga (lines 1 and 2). Rather, the key assumption that the research discussed in the last paragraph called into question was the assumption that parasitic interactions always evolve toward symbiosis (this is discussed in the first 6 lines of the final paragraph, which are lines 45 through 50). As a result, we know that (A) is identifying the wrong assumption that the evidence in the final paragraph overturns, so we can get rid of this answer choice.
(B) says, "to show that findings based on new research regarding fungus classification have implications that affect a long-standing assumption of evolutionary science."
(B) matches up really well with our summary of the final paragraph, which is above. We know that the last paragraph deals with discussing the implications of the research into the DNA of the lichen-forming fungi discussed in the passage. We also know that the research is challenging the assumption that all parasitic interactions always evolve toward a symbiotic relationship (this is discussed in lines 45 through 50). Since this answer choice discusses the implications of research that affect a long-standing assumption of evolutionary science, we know it's the correct answer choice.
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