We can't judge what he really meant by doing so until we know all the _____. [ ]

A. circumstances

B. guess

C. memory

D. judgment

下列程序的输出结果是______。
#include ＜iostream＞
using namespace std;
void fun(int &rf)

rf*=2;

int main()

int num=500;
fun(num);
cout＜＜num＜＜endl;
return 0;