If Augustus secured his borders and refrained from a plan of expansion, why did legions continue to ...

If Augustus secured his borders and refrained from a plan of expansion, why did legions continue to campaign beyond the frontiers of Rome?
 
  A) The campaign beyond frontiers was solely for the purpose of deterring or punishing the attackers on the other side.
  B) The campaigns were used to force trade with those beyond Roman frontiers.
  C) These were the actions of commanders seeking glory, and not adhering to the edict of Augustus.
  D) These campaigns were strictly aimed at recovering captured or killed Roman soldiers.
  E) The campaigns enabled Rome to scout the activities of the barbarians in hopes of preventing incursions.