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.