Why do many United States historians consider the Civil War to be the first “modern” war in American history?

Why do many United States historians consider the Civil War to be the first “modern” war in American history?
A. The Union used divide and conquer techniques.
B. It was the first one fought with guns and cannons.
C. New technology such as telegraphs, iron-plated warships, and mass-produced weapons were common.
D. Vastly improved military tactics led to less-frequent battles and fewer battlefield dead and wounded.