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( )。