What was the main military strategy used on the western font

Trench warfare in World War I meant that the armies dug into trenches to hold their ground.  But it proved impossible for them to make any advances against each other.   If they would try to mount an attack, venturing into "no man's land" between the trenches, they'd get mowed down by machine gun fire.   In addition to bullets and artillery fire, trench warfare also came to involve the use of chemical weapons like mustard gas.  Trench warfare in World War I was miserable and gruesome.  

offline recommendation systems option b

because recommendation systems produce a list of two ways

1. through collaborative filtering

or

2. through content-based filtering