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.