What is one theme found in Walt Whitman's "I Hear America Singing"?

I Hear America Singing
by Walt Whitman

I hear America singing, the varied carols I hear,
Those of mechanics, each one singing his as it should be blithe
and strong,
The carpenter singing his as he measures his plank or beam,
The mason singing his as he makes ready for work, or leaves off
work,
The boatman singing what belongs to him in his boat, the
deckhand singing on the steamboat deck,
The shoemaker singing as he sits on his bench, the hatter singing
as he stands,
The wood-cutter's song, the ploughboy's on his way in the
morning, or at noon intermission or at sundown,
The delicious singing of the mother, or of the young wife at
work, or of the girl sewing or washing,
Each singing what belongs to him or her and to none else,
The day what belongs to the day—at night the party of young
fellows, robust, friendly,
Singing with open mouths their strong melodious songs.

A.
Work and entertainment should not be mixed.
B.
Diverse people contribute to the country's identity.
C.
People are more productive when they enjoy their work.
D.
Working in a modern society allows people to have diverse skills.