读下列程序,对类型转换操作正确的是()。class A{}class B extends A{}class C extends A{}Ao1 = new A();Ao2 = new B();Ao3 = new C();"
A."A a=01;B b=o2;C c =o3;"
B."A a =(A)o1;B b= (B)o2;C c= (C)o3;"
C."A a= (A)o1;B b= (B)o3;C c= (C)o2;"
D."A a= (A)o2;B b= (B)o1;C c= (C)o3;"