The Children's Crusade as an alternative title to the novel is appropriate because, Choice D; The men who fought in war left behind their children to fight a noble cause.
Slaughterhouse-Five is a story which gives an account of Billy Pilgrim's capture and incarceration by the Germans during the last years of World War II, and scattered throughout the narrative are episodes from Billy's life as represented before and after the war.
On this note, the Choice D makes the Children's Crusade most appropriate as an alternative title.
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