B. Because even with many issues that plagued the Roman Empire economically and politically, By the later years of the empire the Roman military was mainly comprised of trained Barbarian soliders who were inferior to earlier Roman soliders causing a significant decrease in the Roman Empires's Military capablities so many barbarian tribes that began to grow such as the Huns, Vandals, Goth etc. Put immense pressure on the Empire, driving them into the ground economically and causing many issues within the government and eventually being sacked, thus marking the end of the Roman Empire. Sorry for the wall of text but hope this helped :)
