以下程序的输出结果为( ) #include using namespace std; class base{ public: virtual char* show()const=0; }; class d1 : virtual public base{ public: char* show() const{return 'd1';} }; class d2: virtual public base{ public: char* show() const{return 'd2';} }; class m:public d1,public d2{ public: char* show()const { return d2::show(); } }; int main() { m m1; cout< return 0; }