有以下程序： #include ＜iostream＞ #include ＜math＞ using namespace std; class point { private: double x; double y; public: point(double a,double b) { x=a; y=b; } friend double distance(point a,point b) ; }; double distance(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 pl(1,2); point p2 (5, 2); cout＜＜distance (pl,p2) ＜＜end1; return 0; } 程序运行后的输出结果是( )。