Why did the western front barely move throughout world war 1?

In the western front, they used Trench Warfare Method 

In Trench Warfare the opposing teams would build some sort of trench surrounding their defended area to prevent the the opposing team in invading their protected Area. It's best used as a Defensive method which require the armies to stay hold and defended their position. That's why they're not moving so much 

D. continental rift

The answer is c) people evacuated but almost all returned