For thousand of years millions of men have labored to clear the forests, to drain the marshes, and to open up highways by land and water. Every inch of soil we cultivate in Europe has been watered by the sweat of several races of men. Every acre has its story of enforced labor, of intolerable toil, of people's sufferings. Every mile of railway, every yard of tunnel, _________.
What is the most critical completion of the passage?
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