glBegin(GL_QUAD_STRIP);
    //glColor3f(1.0, 0.0, 0.0);
        for (angle = 0.0; angle <= 2.01*PI; angle += (2 * PI /N))
        {
            glVertex3f(15.0*cos(angle), 0.0, 15.0*sin(angle));
            glVertex3f(17.0*cos(angle), 0.0, 17.0*sin(angle));
        }
        //glColor3f(0.0, 1.0, 0.0);
        for (angle = 0.0; angle <= 2.01*PI; angle += (2 * PI / N))
        {
            glVertex3f(15.0*cos(angle), 0.0, 15.0*sin(angle));
            glVertex3f(15.0*cos(angle), 4.0, 15.0*sin(angle));
        }
        //glColor3f(0.0, 0.0, 1.0);
        for (angle = 0.0; angle <= 2.01*PI; angle += (2 * PI / N))
        {
            glVertex3f(17.0*cos(angle), 0.0, 17.0*sin(angle));
            glVertex3f(17.0*cos(angle), 4.0, 17.0*sin(angle));
        }
        //glColor3f(1.0, 1.0, 0.0);
        for (angle = 0.0; angle <= 2.01*PI; angle += (2 * PI / N))
        {
            glVertex3f(15.0*cos(angle), 4.0, 15.0*sin(angle));
            glVertex3f(17.0*cos(angle), 4.0, 17.0*sin(angle));
        }
        glEnd();