13.下列何者可在頸後三角(posterior triangle of the neck)內觀察到？ (A)顏面動脈(facial artery)由頸外動脈發出 (B)表淺處之皮膚底下即是迷走神經(vagus nerve) (C)頸神經叢的分支(branches of cervical plexus) (D)肩胛舌骨肌上腹(superior belly of omohyoid muscle)