Final answer:
The Battle of Antietam is significant because it indicated the failure of the Confederacy's invasion of the North, resulted in significant casualties, and led to General Lee's retreat into Virginia.
Step-by-step explanation:
The Battle of Antietam is significant because it indicated that the Confederacy's invasion of the North had failed, as General Lee's army lost a quarter of its men and retreated into Virginia. Additionally, more than 20,000 soldiers were killed or wounded over the course of one day. Therefore, the correct answer is option D: all of the above.
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