Trial and error experience showed that "C" - using a dual flanking maneuver - was the most successful tactic. To destroy a concrete pillbox you would need a massive artillery shell with a direct hit. The seige guns the Canadians had were large enough, but not that accurate. A near miss would cause little damage. Aerial bombing simply wasn't sufficiently destructive. A 60 or 120 kg bomb lacked the power to blast through reinforced concrete. Rifle grenade had even less explosive power against fortifications. Only a lucky shot through an opening would work. The Canadians learned that you needed two sets of attackers, one on each side of the pillbox. When the German guns swung one way, the soldiers on the other side advanced, and vice versa. Eventually one or both flanking attacks could work around behind the pillbox and attack.