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)<<endl;return 0; } 执行结果是
A.20
B.30
C.900
D.400