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According to paragraph 4, which of the following is true of Britain's auxiliary regiments of the Roman army? A. Membership in these regiments reached its highest point during the region of the emperor Hadrian. B. Most of the units recruited in Britain were sent to Morocco and other stations outside Europe. C. Soldiers served in the regiments for many years and after retirement generally stayed where they had been stationed. D. Most of the regiments stationed on the frontier were new units transferred from a neighboring province.