有以下程序： class Date { public: Date(int y, int m, int d); year = y; month = m; day = d; Date(int y = 2000) year = y; month = 10; day = 1; Date(Date &d) { year = d.year; month = d.month; day = a.day; } void print() cout＜＜year＜＜"."＜＜month＜＜"."＜＜day＜＜end1; } private: int year, month,day; };Date fun(Date d) Date temp; temp = d; return temp; } int main() { Date date 1 (2000,1,1),date2(0,0,0); Date date3(date 1); date2 = fun(date3); return 0; } 程序执行时，Date 类的拷贝构造函数被调用的次数是
A．2
B．3
C．4
D．5
