单项选择题

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()

A.1  
B.5   
C. 4  
D. 6