问题：

[单选]

23.int z=5； 
24. 
25. public void stuff1（int x） { 
26. assert （x> 0）； 
27. switch（x） { 
28. case 2： x= 3； 
29. default： assert false； } } 
30. 
31. private void stuff2（int y） { assert （y < 0）； } 
32. 
33. private void stuff3（） { assert （stuff4O）； }
34. 
35. private boolean stuff4（） { z = 6； return false； } 
Which is true？（） 
 

A .  All of the assert statements are used appropriately.
B .  Only the assert statement on line 31 is used appropriately.
C .  The assert statements on lines 29 and 31 are used appropriately.
D .  The assert statements on lines 26 and 29 are used appropriately.
E .  The assert statements on lines 29 and 33 are used appropriately.
F .  The assert statements on lines 29， 31， and 33 are used appropriately.
G .  The assert statements on lines 26， 29， and 31 are used appropriately.