[说明]
下面程序定义了一个类Shape及其成员函数，然后从类Shape派生出类Point、类Circle、类Cylinder，以及它们的成员函数。
[Java程序]
(1) Shape
public abstract void printShapeName(); //纯虚函数。
public double area()return 0.0;
public double volumn()return 0.0;

class Point extends Shape
private double x.y;
public Point (double a, double b)x=a; y=b;
public void setPoint (double a,double b)x=a;y=b;
final public double getX() return x;
final public double getY()return y;
public void printShapeName()
(2) ;


class Circle extends Point
private double radius;
public Circle(double r, double a, double b)
(3) ;
radius=r＞0 r:0;

public void setRadius (double r)
radius=r＞0 r:0;

public double getRadius () return radius;
public double area()return 3.14259 * radius * radius;
public void printshapeName ()
System.out.print ("circle: ") ;
;
;
class Cylinder (4)
private double height;
public Cylinder(double h, double r, double x, double y)
super (r, x, y) ;
height = h＞0 h:0;

public void setHeight (double h)
height = h＞0 h:0;

public double area () //计算表面积
return (5) ;

public double volumn ()
double r=super.getRadius () ;
return 3.14159 * r * r * height;

public void printShapeName ()
System. out .print ("cylinder: ") ;

根据你对各类天体系统的认识，判断下列各选项中属于天体系统的是（）

A.星云

B.月球

C.仙女座

D.太阳系