下列关于中国民族工业发展历程的叙述，正确的是（  ）

   A lift is wonderful. It is really only a small room. Rooms usually stay in one place. Lifts travel up and down all day long.
    Sometimes a worker stands in the lift. He or she operates it up and down. In modern lifts there is no worker.  People walk in. They know which floor they want. They push a button and the lift goes to that floor. It is all very fast and easy.
   Lifts are very important to us. Why? Think about a tall building. Maybe it has twenty floors. Maybe it has fifty or more. Who can walk up all the stairs? Maybe people can climb them one time. Can someone climb thirty floors to an office every day? Can small children walk up to their room on the twenty-fourth floor? Can their mothers and fathers carry food up all those stairs? Of course not.
   We can have high buildings because we have lifts. We could not have all the beautiful tall buildings in the world without lifts. They are really wonderful.

一个圆锥的母线长是9，底面圆的半径是6，则这个圆锥的侧面积是(   )