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.K; } 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