Given:
1. import java.util.*;
2. public class Ps {
3. public static void main(String[] args) {
4. PriorityQueue＜String＞ pq = new PriorityQueue＜String＞();
5. pq.add("4");
6. pq.add("7");
7. pq.add("2" );
8. // insert code here
9. } }
Which code fragment(s), inserted independently at line 8, produce the output "2-4-7-" (Choose all that apply.)

单项选择题Your virtual-world simulator is designed such that Grobnets can have several Ooflas, and Yazells support the contract implied by Whompers. Grobnets are a type of Yazell, and can also act as Floordums or Whompers. Which of the following code fragments legally represent this design (Choose all that apply.)

多项选择题Given:2. public class Checkout2 implements Runnable {3. void doStuff() { }4. synchronized void doSynch() {5. try {Thread.sleep(1000); }6. catch (Exception e) {System.out.print( e ); }7. }8. public static void main(String[] args) {9. long start = System.currentTimeMillis();10. new Thread(new Checkout2()) .start();11. Thread t1 = new Thread(new Checkout2());12. t1.start();13. try {t1.join();}14. catch (Exception e) {System.out.print( e ); }15. System.out.print in ( elapsed: + (System.currentTimeMillis() - start)); 16. }17. public void run() {18. for(int j = 0; j ＜ 4; j++) {19. doStuff();20. try {Thread.sleep(1000); }21. catch (Exception e) {System.out.print( e ); }22. doSynch();23. } } }Which are true (Choose all that apply.)