单项选择题

有以下程序#include <iostream>using namespace std;int a;int fun ();int main (){ extern int a; int b; a = 100; b = fun(); cout<<b<<end1; return 0;}int fun{){ extern int a; return (10*a ;} 其程序运行后的输出结果是

A.100
B.10
C.1000
D.10000