1. class TestA { 
2. TestB b； 
3. TestA（） { 
4. b = new TestB（this）； 
5. } 
6. } 
7. class TestB { 
8. TestA a； 
9. TestB（TestA a） { 
10. this.a = a； 
11. } 
12. } 
13. class TestAll { 
14. public static void main （String args[]） { 
15. new TestAll（）.makeThings（）；
16. // ...code continues on 
17. } 
18. void makeThings（） { 
19. TestA test = new TestA（）；
20. } 
21. } 
Which two statements are true after line 15， before main completes？（）

A. Line 15 causes a stack overflow.
B. An exception is thrown at runtime.
C. The object referenced by a is eligible for garbage collection.
D. The object referenced by b is eligible for garbage collection.
E. The object referenced by a is not eligible for garbage collection.
F. The object referenced by b is not eligible for garbage collection.