单选题 有如下程序：#include＜iostream＞using namespace std；class Base{private：void fun1()const{cout＜＜"fun1"；}protected：void fun2()const{cout＜＜"fun2"；}public：void fun3()const{cout＜＜"fun3"；}}；class Derived：protected Base{public：void fun4()const{cout＜＜"fun4"；}}；int main(){Derived obj；obj．fun1()；／／①obj．fun2()；／／②obj．fun3()；／／③obj．fun4()；／／④return0；}其中有语法错误的语句是( )。