单选题 有如下程序： #include＜iostream＞ using namespace std; class Base { pubfic: Base(){cout＜＜"BB";f();} void f(){cout＜＜"Bf":} }; class Derived:public Base { public: Derived(){cout＜＜"DD";) voidf(){cout＜＜"Df":} }; intmain(){Derived d;retum 0;} 执行上面的程序将输出