Why was Antietam the deadliest one day battle of the Civil War? Give two reasons please :(

Fighting began in the foggy dawn hours of September 17. As savage and bloody combat continued for eight hours across the region, the Confederates were pushed back but not beaten, despite sustaining some 15,000 casualties.

By the time the sun went down, both armies still held their ground, despite staggering combined casualties–nearly 23,000 of the 100,000 soldiers engaged, including more than 3,600 dead. McClellan’s center never moved forward, leaving a large number of Union troops that did not participate in the battle.

i am confused, what is the question? i can you out if you put it into more depth.