单选题 C#中下列代码运行结果是（） public class Student { public virtual void Exam() { Console.WriteLine("学生都要考试"); } } public class Undergraduate : Student { public new void Exam() { Console.WriteLine("大学生有选择考试科目的权利"); } } class Program { static void Main(string[] args) { Student stu = new Undergraduate(); stu.Exam(); } }