public SingleObject create() {
      Random rand=new Random();
      double num=rand.nextDouble();
      int pos=rand.nextInt(stage.getWidth());
      if(cnt%2==0 && grade=='f'){
         stage.getSound("audio/missile_fly.wav").play();
      }
      if(grade=='f'){
         cnt++;
      }
      return new SingleObject(pos, 0, (Math.PI/4.0)*(num-0.5), frequency/stage.fps/2.0,image);
   }