9．小肠与其吸收功能相适应的结构特点是

①小肠长5―6米                                              ②肠壁上有肠腺

③有小肠绒毛                                                 ④小肠绒毛内有毛细血管

A．①②④                B．①③④                   C．②③④                   D．①②③