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  
}  
}