单项选择题

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 = d.day; } void print() { cout＜＜year＜＜"."＜＜month＜＜"."＜＜day＜＜endl; } private: int year,month,day; }; Date fun(Date d) { Date temp; temp = d; resurn temp; } int main () { Date date1 (2000,1,1),date2 (0,0,0); Date date3 (date1); 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

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