问一个面对对象中的问题，在线等

class Building(){ public class Barn extends Building{
      Building build1=new Building();
      Barn barn1=new Barn();
       Barn baen2=new Barn();//10
      Object obj1=(Object)build1;//11
      String str1=(String)build1;//12
     Building build1=(Building)barn1;//13

}
}a   如果第10行删除 the compilation succeds
b   如果第11行删除 the compilation succeds
c   如果第12行删除 the compilation succeds
d   都不能删除