单项选择题

下面程序的输出结果是( )。 #include＜iostream＞ #include＜math.h＞ using namespace std; class point { private: double x; double y; public: point(double a,double b) { x=a; y=b; } friend double distances(point a,point b); }; double distances(point a,point b) { return sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y)); } int main() { point p1(1,2); point p2(5,2); cout＜＜distances(p1,p2)＜＜end1; return 0; }