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.
Well some rivers in Europe are used to transport goods. Sometime people even travel on them. River also can provide power.
Answer:
Life is all about taking risks.