正在下载：人教版九年级英语下册综合复习试题及答案3套

五、单项选择。(每小题1分，共计15分) ( )21.—Is this ________ useful book about health? —Yes，many famous doctors in it encourage us to take ________ active part in sports. A．a；/     B．an；/     C．an；an    D．a；an ...