The best explanation why the area between Spain and Gaul was not part of the Roman empire until that later period was a culmination from both undesireability and difficulty in accessing this region due to its mountaineous nature. So I would wager the answer being both C and D - undesireability and difficulty in access.
Industrialization and/or Urbanization. We now have technology fields, and higher paying job fields, that seem more interesting or grabs the younger generations attention.
Answer:
The Ming practiced isolationism to prevent foreign influences from undermining their values and society.
The French who had direct contact with the Americans were able to successfully implement Enlightenment ideas into a new political system. The National Assembly in France even used the American Declaration of Independence as a model when drafting the Declaration of the Rights of Man and the Citizen in 1789
