One of the Key Developments Enabling the Union to Stop

One of the key developments enabling the Union to stop the Confederate thrust into the North at Antietam was

A) Europe's refusal to help the South before the battle.
B) the Union's discovery of Robert E. Lee's battle plans.
C) Lincoln's removal of General McClellan from his command.
D) the use of the new repeating rifle for the first time.
E) the death of Stonewall Jackson during the battle.