请完成如下程序class Shape { protected int width, height; public Shape( int a=0, int b=0) { width = a; height = ______ ; } public ______ int area() { Console.WriteLine("父类的面积："); ______ 0; }class Rectangle: Shape{ public Rectangle( int a=0, int b=0): base(a, b) { } public override int area () { Console.WriteLine("Rectangle 类的面积："); return (width * height); } } class Triangle: Shape{ public Triangle(int a = 0, int b = 0): ______ (a, b) { } public ______ ______ area() { Console.WriteLine("Triangle 类的面积："); return (width * height / 2); } }class Caller { public void CallArea( ______ sh) { int a; a = sh.area(); Console.WriteLine("面积： {0}", a); } } class Tester{ static void Main(string[] args) { Caller c = new Caller(); Rectangle r = new Rectangle(10, 7); Triangle t = new Triangle(10, 5); c.CallArea(r); c.CallArea(t); Console.ReadKey(); } } }Rectangle 类的面积： 面积： 70 Triangle 类的面积： 面积： 25