Answer choice (C) very clearly follows the trajectory of the passage. First with definitions of lichens, which occurs on line 1, then to the discussion of the difficulty of classifying their fungal components, which can be found starting line 15, then to the resolution of this difficulty and implications of the resulting research starting on line 32 to the end of the passage.
Answer choice (A), on the other hand, begins with an "explanation of the difficulty of classifying lichens." The EXPLANATION--as opposed to the first mention--of why classifying lichens is difficult can be found on line 15.
So, right off the bat, answer choice (A) has skipped over the first paragraph. The second part of answer choice (A) states: "description of the DNA-sequence of lichen-forming fungi." The passage never DESCRIBES the DNA sequence of lichen-forming fungi. It merely states that researchers have been able to use new analytical tools "that allow them to isolate the DNA of fungi in parasitic or symbiotic relationships," and what this information means (32-58).
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