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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def activities(self):
 """ Access the activities :returns: twilio.rest.taskrouter.v1.workspace.activity.ActivityList :rtype: twilio.rest.taskrouter.v1.workspace.activity.ActivityList """

        if self._activities is None:
            self._activities = ActivityList(self._version, workspace_sid=self._solution['sid'], )
        return self._activities