Answer:
a)The new manufacturing capacity of European nations led to massive military buildups.
Step-by-step explanation:
B doesn't make sense becuase there wass build up and the war was only so bad because the two sides were prettty evenly matched.
C does'nt make sense becuase the US didn't attack and start WW1 the asisnation of archduke ferdand and subsiquent Prussian invasion
Thus A is the only one that makes sense