1. import java.util.*； 
2. public class Example { 
3. public static void main（String[] args） { 
4. // insert code here 
5. set.add（new integer（2））； 
6. set.add（new integer（l））； 
7. System.out.println（set）； 
8. } 
9. } 
Which code， inserted at line 4， guarantees that this program will output [1，2]？ （）

10. interface Jumper { public void jump（）； }
 ...... 
20. class Animal {} 
...... 
30. class Dog extends Animal { 31. Tail tail； 32. } 
...... 
40. class Beagle extends Dog implements Jumper { 
41. public void jump（） { } 
42. } 
....... 
50. class Cat implements Jumper { 
51. public void jump（） { } 
52. } 
Which three are true？（）