单选题 The brachial plexus A、enters the axilla anterior to the scalenus anterior muscle B、has no branches from the trunks C、has radial and axillary nerves that are formed from poste-rior primary rams D、is enclosed in a fascial sheath derived from the preverte-bral fascia E、has four major terninal branches：the radial，the ulnar，the axillary and long thoracic nerves