单项选择题 Which statements， when inserted at the indicated position in the following code， will cause a runtime exception when attempting to run the program？（）   class A {}   class B extends A {}   class C extends A {}   public class Q3ae4 {   public static void main（String args[]） {   A x = new A（）；   B y = new B（）；   C z = new C（）；   // insert statement here   }   }

单项选择题 WhichmethodsfromtheStringandStringBufferclassesmodifytheobjectonwhichtheyarecalled？（）

单项选择题 Which is the earliest line in the following code after which the object created on the line marked （0） will be a candidate for being garbage collected， assuming no compiler optimizations are done？ （）  public class Q76a9 {   static String f（） {   String a = "hello"；   String b = "bye"； // （0）   String c = b + "!"； // （1）   String d = b；  b = a； // （2）   d = a； // （3）   return c； // （4）  }   public static void main（String args[]） {   String msg = f（）；   System.out.println（msg）； // （5）   }   }