challenge: What was the primary purpose of building Fort de Bois-d'Arcy between 1874 and 1881?
  
  A. To reinforce the western fortifications of Paris and cover the approach of a relieving army
  B. To reinforce the eastern fortifications of Paris and protect the city from invasion
  C. To provide a strategic outpost for French military operations in Western Europe
  D. To serve as a military training facility for the French army
  Answer: 