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Why does the author mention "mature floodplain tropical forest in the Amazon River basin" in the passage? A. To dispute the idea that tropical forests are arranged in layers B. To give an example of the complex architecture vegetation displays in a dense area C. To suggest that the layers of canopy in some tropical forests can exceed the usual three or four D. To emphasize that the layers of canopy in a tropical forest give evidence of the number of layers of root tissues below the ground