I'm going to drill down each choice to what I think is its most significant element:
A. helped shorten the war - False; the entry of the United States helped shorten the war, and of course the mechanized weaponry, most of which the US brought.
B. higher casualties - Definitely. Trench warfare is essentially man to man combat. The job of the armed forces is to k**l people and b**w up stuff. You can't realistically b**w up holes in the ground, aka trenches, so the only thing that leaves is to k**l people.
C. fewer casualties - Fewer than what? No way; trench warfare was one of the very origins of the term cannon fodder
D. less trauma for soldiers - No; they used a lot of mustard gas and fragmentation ordnance. Also, medicine wasn't very modern back then.
My answer would be B.
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