When the Confederacy lost Vicksburg and control of the Mississippi River, the union was essentially cut into the half. Option B is correct.
Why Mississippi River cut into the half?
The Mississippi River was designated as the new nation's western border by the Treaty of Paris, which ended the Revolutionary War and established American independence in 1783.
The Louisiana Purchase, signed by President Thomas Jefferson in 1803, considerably increased the size of the country by adding land west of the Mississippi River.
As a result of that division, the supply chain of Mississippi water was destroyed.
Therefore, option B is correct.
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