int sun（int x，int y）                                                                             {                                                                                                     return x + y                                                                                      }                                                                                                   int main（int argc，char*argv[]）    //这句是什么意思呢？                                           {                                                                                                       int ret；                                                                                           ret = sun（10,20）；  //这句话怎么理解 赋值的是多少呢                                               printf（"结果：%d\n"，ret）；   // 这段也看不懂，尤其是  %d\n                                       return 0； 最后返回为什么必须是0呢？换个数不行吗？                                              }