How does the repetition of the word singing influence the overall meaning of the poem “i hear america singing” by walt whitman?

The repetition of the work "singing" is associated with the enumeration of who is singing: for example the carpenter, the mason and the shoemaker. 
Therefore, "singing" contributes to the feeling that everyone is singing: it  provides a feeling of unity. 
This singing also symbolizes hope for well-being: all those singing people have a job and a secure existence. 
The correct answer is: The repetition emphasizes hope and a unified vision for America.

Can you give the sentences?