有如下程序: #include usingnamespacestd; classB{ public: virtualvoidshow(){cout$amp; classD:publicB{ public: voidshow(){cout$amp; }; voidfun1(B*ptr){ptr->show(); }voidfun2(B&ref){ref.show(); }voidfun3(BB){b.show(); } intmain(){ Bb,*p=newD; Dd; funl(p); fun2(b); fun3(d); return0; } 程序的输出结果是( )。