求高手能解这道题的！给分40分！


    public class Person
    {
        private string _personName;
        public string PersonName
        {
            get;
            set;
        }
        private int _age;
        public int Age
        {
            get;
            set;
        }
        public Person()
        {
        }
        public Person(string personname, int age)
        {
            this._personName = personname;
            this._age = age;
        }
        public virtual void WriteInfo()
        {
            Console.WriteLine(this._personName + ":" + this._age);
        }
    }
    public class Teacher : Person
    {
        private string _university;
        public string University
        {
            get;
            set;
        }
        public Teacher() : base() { }
        public Teacher(string university):base("张三", 30)
        {
            this._university = university;
        }
        public virtual void WriteInfo()
        {
            Console.WriteLine(this._university);
        }
    }使用
            Person person = new Person();
            person.Age = 100;
            person.WriteInfo();
            person.PersonName = "李四";
            person = new Person("王五", 40);
            person.WriteInfo();
            Teacher teacher = new Teacher();
            teacher.University = "北京大学";
            teacher.Age = 50;
            teacher.PersonName = "赵六";
            teacher.WriteInfo();
            teacher = new Teacher("清华大学");
            teacher.WriteInfo();