Final answer:
America's carols are 'varied' because each person has a unique singing voice, which reflects the diversity of people and their experiences. It is the differences in individual voices that contribute to this variety, creating a collective sound that represents the nation's multitude of workers and personalities.
Step-by-step explanation:
The varied carols in America mentioned in the poem “I Hear America Singing” refer to the different kinds of work and the unique voices of the people who perform them. This diversity is celebrated and reflected through the individual songs, or carols, that each worker sings. The answer to why America's carols are 'varied' is D. because people have different singing voices.
The human voice is an incredibly powerful tool in music because it can convey a wide range of emotions, nuances, and expressions that are unique to each individual. Just as workers may take pride in their different roles and tasks, their singing voices represent their personalities, emotions, and experiences leading to a rich tapestry of sounds.
This accounts for the 'varied' aspect of America's carols; it is not merely about the visual differences or the skill levels of the singers, but the inherent diversity in each person’s voice that contributes to the overall variety of carols sung across America.