The two advantages the Union had over the Confederacy during the Civil War are:
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Larger population - The Union had a significantly larger population than the Confederacy, which provided more soldiers for the army and a larger workforce for war-related industries.
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Extensive railroad system - The Union had a more developed railroad network, which allowed for better transportation of troops, supplies, and resources throughout the war.
The other options listed, such as "better leadership under Robert E. Lee" and "greater knowledge of the terrain," were advantages for the Confederacy. "More farmland" was not an advantage for the Union relative to their needs in the war context.