A.头部应稍后仰，张口时下颌（牙合）平面与地面平行
B.头部应稍后仰，张口时下颌（牙合）平面与地面成45°
C.头部应稍后仰，张口时下颌（牙合）平面与地面成90°
D.头部应稍后仰，张口时下颌（牙合）平面与地面成60°
E.头部应稍后仰，张口时下颌（牙合）平面与地面成30°