以下程序的执行结果是 ( )。
#include ＜iostream＞
using namespace std;
class sample
private:
int x;
public:
sample (int

A．

x=a;

friend double square(sample s);
;
double square(sample s)

return s.x*s.x;

int main ( )

sample s1(20) ,s2(30);
cout＜＜square (s2) ＜＜end1;
return 0;
A) 20
B．30
C．900
D．400