下面程序运行的结果是（）public delegate void BuyTicketDelegate(string num); public class Student { public event BuyTicketDelegate btEvent; private string num; public Student(string Num) { this.num = Num; Console.WriteLine("student"); btEvent += new BuyTicketDelegate(this.BuyTicket); } private void BuyTicket(string num) { Console.WriteLine(num ); } public void ShowNum() { Console.WriteLine("show"); if (btEvent!=null) btEvent(num); } }class Program { static void Main(string[] args) { Student stu = new Student("123"); stu.ShowNum(); } }