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Anaconda Plan :) Understand more here: http://en.wikipedia.org/wiki/Anaconda_plan
C is the right answer, it seems to refer to some point or points that would give control of the Mississippi River, which divided the western part of the Confederacy, particularly the large and wealthy state of Texas, from the rest. It should be remembered that in the 19th century, rivers were still the most important means of transportation. Control of the Mississippi would allow the Union to strangle the commerce of the South, move its own troops and intelligence faster, and restrict the equivalent moves of the South. It looks like C is in the neighborhood of Memphis, although it may not refer to any specific spot. Memphis was in fact taken by the Union in June of 1862, but the final struggle for control of the Mississippi took place at Vicksburg the following summer, and it was Grant's victory there that (along with Lee's defeat at Gettysburg) convinced the North it could win the war, secured the re-election of Lincoln, and convinced Lincoln to bring Grant eastward to take command of the main efforts of the Union to subdue the South.