&3.以下程序是:由长方形类派生出正方形类,正方形类的构造方法调用其基类的构造方法。请补充空白处代码: static void Main(string[] args) { Console.WriteLine("请输入正方形的长:"); Square s = new Square(double.Parse(Console.ReadLine())); Console.WriteLine("正方形的面积为:{0}", s.Area()); Console.ReadKey(); } } class Rectangle { protected double length; private double width; public Rectangle(double l, double w) { length = l; width = w; } } class Square : Rectangle { public Square(double l) : { } public double Area() { return length * length; } }