单选题 有如下程序：#nclude＜iostream＞using namespace std；class Base{public：void fun(){cout＜＜"Base：：fun"＜＜endl；}}；class Derived：public Base{public：void fun(){____________________________cout＜＜"Derived：：fun"＜＜endl；}}；int main(){Derived d；d．fun()；return0；}已知其执行后的输出结果为：Base：：funDerived：：fun则程序中下画线处应填入的语句是( )。