有以下程序：
#include ＜iostream＞
#include ＜math＞
using namespace std;
class pointprivate:
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. x -b. x)*(a. x-b. x));int main () point p1(1,2);
point p2(5,2);
cout＜＜distance (p1, p2)＜＜end1;
return 0; 程序运行后的输出结果是
A．1
B．5
C．4
D．6
