Final answer:
The best evidence for the Eastern Front being the worst place for soldiers is B, highlighting the lengthy marches soldiers endured, reflecting the tough conditions and high mobility that characterized the Eastern Front.
Step-by-step explanation:
The most compelling piece of evidence to support the claim that the Eastern Front of WWI was the worst place for soldiers to fight is found in the description of the high mobility of the conflict on this front, which contrasts the static trench warfare seen in the West. Russian soldiers, despite early successes, suffered from an inadequate industrial base, shortages in munitions, and occasionally incompetent commanders. These factors, combined with having to endure long marches across vast distances to engage in battles, support the selection of this evidence.
Therefore, the best piece of evidence to support the stated claim would be:
B. The soldiers had to march for 100 miles between battle sites.
This statement highlights the arduous conditions the soldiers faced due to the highly mobile nature of the warfare on the Eastern Front, potentially making it the worst place to fight during World War I.