The human river dwindles when 'tis past the hour of eight, its waves go flowing faster in the fear of being late; but slowly drag the moments, whilst beneath the dust and heat the city grinds the owners of the faces in the street— grinding body, grinding soul, yielding scarce enough to eat— oh! i sorrow for the owners of the faces in the street.
In the line "the human river dwindles when 'tis past the hour of eight," how does the poet use the phrase "human river" to create a layer of meaning?

a. The phrase refers to a real river near the city.
b. The phrase metaphorically portrays the flow of people in the street.
c. The phrase highlights the beauty of the city.
d. The phrase emphasizes how happy the people are.