B. American labourers are all different and sing their own song
The repetition of "singing" in Walt Whitman's "I Hear America Singing" conveys that American laborers each have their distinct identities and contributions, emphasizing individualism within a collective identity. This supports the idea that while they are part of a unified whole, each laborer expresses their own unique song. Therefore, the correct option is B : American laborers are all different and sing their own individual song.
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