面试题抉择，求解释，谢了

面试题选择，求解释，，，，谢了
7、Given the folowing classes which of the following will compile without 
error? 
  interface IFace{} 
  class CFace implements IFace{} 
  class Base{} 
  public class ObRef extends Base{ 
  public static void main(String argv[]){ 
  ObRef ob = new ObRef(); 
  Base b = new Base(); 
  Object o1 = new Object(); 
  IFace o2 = new CFace(); 
  } 
  } 
  A. o1=o2; B. b=ob; C. ob=b; D. o1=b; 


------解决方案--------------------
刚刚看的不仔细，正确应该是 A B D ，A是因为所有类都默认继承自Object类，所以也在同一个继承树上，D也在同一个继承树上，是将子类实例赋值给父类的父类，刚刚没看好，呵呵