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Must be True. The correct answer choice is (A)
The stimulus consists of premises, but drives toward a fill-in-the-blank conclusion. You must infer the main point of the argument.
The premises establish that mature white pines obstruct sunlight well enough that such pines cannot regenerate in their own shade, and you need to decide what is likely in a stand of white pines in a dense forest.
You are supposed to pick up on the theme of sunlight-obstruction, and realize that as a stand of white pines becomes thicker, it becomes less likely that new white pines will grow. That would suggest that a dense stand of white pines consists of trees of fairly similar ages.
Answer choice (A): This is the correct answer choice. At the very least, within the stand the pines should not differ drastically in age. According to the stimulus, younger white pines would not survive if they had to grow under the cover of older white pines, so you would expect that a close grouping of white pines consists of trees of fairly similar ages.
Answer choice (B): The area would not have to be cleared of all trees, just enough trees that the white pine saplings could get enough light to grow. White pines cannot regenerate in their own shade, but might be able to regenerate in a lesser amount of shade.
Answer choice (C): The stimulus suggests that older white pines block the sunlight to seedlings, but there is no reason to assume that older white pines would deprive each other of sunlight enough that some of the older trees would die.
Answer choice (D): Since it is quite possible that the obstruction of sunlight makes it impossible for any seedling to succeed in a stand of white pines, it does not follow that other species of trees would colonize and replace the stand of white pines. This choice is unsupported, somewhat contrary, and incorrect.
Answer choice (E): The stimulus does not establish that white pines grow at a fairly invariable rate. Furthermore, the stimulus tends to suggest that the trees are all of very similar ages if they are to survive together, which tends not to support the idea that the height differences should be attributed only to age.