Kyle M. answered 03/28/14
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All-Levels General and Academic English - Speaking, Reading, Writing
You are correct because:
1) the text never mentions "the country," but vaguely refers to "the two sections," which, in this context, merely implies "two sides of the country." However, a greater implication is that this is not the main idea, or it would have been stated more directly. This eliminates choices A & B.
2) this excerpt never discusses Mason & Dixon as historical persons, nor mentions any division between them. This eliminates choice C.
3) this selection, however, does discuss a "great crisis" that resulted from "fires that had long been smoldering," a metaphor implying that a collection of regional conflicts had built up over "generations" & finally broke out into a larger, hotter conflict. The author implies that the Mason-Dixon Line has more significance than as a mere survey boundary, but this is not enough to support "the two sides" or "Mason and Dixon's division" as the main idea. The "great crisis" remains the focus throughout, and everything supports "long-building tensions" as part of the main idea.