Answer:
OB. Those areas had powerful enemy tribes.
The Roman Empire faced many challenges as it attempted to expand its borders, and one of the biggest obstacles was the presence of powerful enemy tribes in Northern Europe, far northern Britain, and Asia. These tribes were often well-organized and well-equipped, and they presented a significant military threat to the Roman army. Additionally, the harsh and unforgiving terrain in these areas made it difficult for the Romans to establish a foothold and effectively control the territory. As a result, the empire was unable to expand into these areas, and they remained outside of Roman control.
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