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{}A o1 = new A();A o2 = new B();A o3 = new C();
A a = o1;B b = o2;C c = o3;
B. 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;