Answer: A.
Explanation: The primary characteristic of the Western Front during World War I, between the Swiss Alps and the English Channel, was trench warfare. The Western Front was marked by an extensive network of heavily fortified trenches and barbed wire extending from the North Sea to the Swiss border. Both opposing sides were entrenched, leading to a brutal and lethal stalemate. This type of warfare involved soldiers residing in unhygienic conditions, being subjected to constant shelling and deploying chemical weapons, and launching assaults across no man's land into enemy positions. The static nature of trench warfare resulted in prolonged and expensive battles, contributing to the high number of casualties sustained by both factions.