21下列是使用C++語言撰寫的程式：#include <iostream>using namespace std;class Employee{public:void display(void) {}string name;};class HourlyEmployee: private Employee{public:void display(void) {}private:double hours;};int main(void){//程式加入點return 0;}下列那一個程式片段在加入main函式之後，編譯時會產生錯誤？