单项选择题

有以下程序
#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;

执行结果是A) 20
B.30
C.900
D.400