问答题

{{*HTML*}}在下面程序横线处填上适当的语句，使其输出结果为0,15,15。
#include ＜iostream.h＞
class base
{ public:
______ f() { return 0; }
};
class derived:public base
{ public:
int a,b,c;
______ set(int x, int y, int z) {a =x; b =y; c =z;}
int f() { return a + b + c ; }
};
void main()
{ base b;
derived d;
cout＜＜b.f()＜＜" ,";
d.set(3,5,7);
cout＜＜d.f()＜＜" ,";
base &p = d;
cout＜＜p.f()＜＜endl;
}