(A) the United States immediately joined the war
(B) ** trench warfare made it difficult for either side to win
(C) the French army was able to push Germany out
(D) the Germany army fought with outdated weapons
EXPLANATIONS BELOW
Concept note-1: -After the buildup of forces in 1915, the Western Front became a stalemated struggle between equals, to be decided by attrition. Frontal assaults, and their associated casualties, became inevitable because the continuous trench lines had no open flanks.
Concept note-2: -The devastating firepower of modern weapons helped create the trench stalemate on the Western Front during the First World War. Armies were forced to adapt their tactics and pursue new technologies as a way of breaking the deadlock.
Concept note-3: -The conventional explanation for why the Western Front in World War I settled into a stalemate is that the power of defensive weapons was stronger than the offensive methods employed.
Concept note-4: -Trechwarfare led to a stalemate because both sides lost too many soilders and any further action would just lead to more and more problems and deadlier consequences.