Why does the author mention that "some of the forts continued in occupation long after Rome ceased to have any formal authority in Britain"? A. To emphasize the degree to which the stability of the British northern frontier depended on firm military control B. To suggest that the Romans continued to occupy Britain even after they had formally given up the right to do so C. To support the claim that forts continued to serve an import economic function even after they ceased to be of any military use D. To describe one of the things that resulted from frontier garrisons' becoming part of the local community over a long period