All of the above is the answer
Explanation:
USA joined WWI because, Germany was a big power, and they sank US merchant ships coming to Britain, and led US to see what's happening.
It was difficult because of the conditions and the little time the soldiers had to dig these massive trenches. Also because the enemy would often build their trench less than a hundred yards away. When the enemy got pushed back they would often turn around the enemies tench and use their trench so they didn’t have to build a new one.
A. Southern and Eastern Europe.