The best answer is "D" - use all three methods. The Vimy area had dozens of caverns and tunnels going back to the Middle Ages and British "sappers" added even more. They became underground barracks, store rooms, command centers and hospitals - al to hide the extent of the build-up from the Germans. As well, virtually all movement and activity was done at night. During the day nothing moved on the surface. Allied fighter pilots - including Billy Bishop - flew patrols to keep nosey German aircraft away.