Read the following passage from "Resistance to Civil Government" by Henry David Thoreau:

If the injustice has a spring, or a pulley, or a rope, or a crank, exclusively for itself, then perhaps you may consider whether the remedy will not be worse than the evil; but if it is of such a nature that it requires you to be the agent of injustice to another, then, I say, break the law. Let your life be a counter-friction to stop the machine.
How does Thoreau use metaphor in this passage?
A. To change the focus of the essay to the reader
B. To compare injustice to something concrete
C. To demonstrate faulty logic in common injustices
D. To trigger strong emotions from the reader