Allusion can be defined as a reference to a person place or thing. For example, if a poem reference a person like Martin Luther King Jr., then the author is making an allusion to Martin Luther King Jr.
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The "Sweet hereafter" is the idiomatic way to refer to heave, so that explains why the correct answer is B <em>"A beautiful place the dead go, like heaven". </em>
In reference to the novel's title, we can see how this is related owing to the fact that the narrator is telling his story after an accident in which he almost die. We can think that his "sweet hereafter" is the second chance the world give him to not die and keep on living.
For it to make sence. Poems have to make sence, they can be strange , but they still make sense.