阅读下列的程序 import java.util.*; class Student { String name ; String age ; public Student ( String name , String age ) { this . name = name; this . age = age; } public String toString () { return name + ":" + age; } } public class Example { public static void main ( String [] args ) { Set set = new HashSet (); set . add ( new Student ( "Tom" , "10" )); set . add ( new Student ( "Jerry" , "10" )); set . add ( new Student ( "Tom" , "10" )); } } set . add ( new Student ( "Tom" , "10" )); } } 下列选项中,程序的运行结果是