A.0 B.1 C.01 D.001
A.类Person是类Undergraduate的基类 B.类Undergraduate从类Student公有继承 C.类Student是类Person的派生类 D.类Undergraduate是类Person的派生类
A.1B B.1D C.2B D.2D
A.BBB B.BBD C.DBB D.DBD