阅读以下说明和C++程序，将应填入(n)处的字句写在对应栏内。

[说明]

下面程序是为汽车市场编制的一个程序的一部分。其中automobile是基类。

[C++程序]

//Auto.h

ifndef AUTO_H

define AUTO_H

class automobile

{

(1):

int miles_per_gallon; //汽车每加仑行驶公里数

float fuel_capacity； //油箱容积

public:

void initialize(int in_mpg,int in_fuel);

int get_mpg(void);

float get_fuel(void);

float travel_distance(void);

}

endif

//Auto.cpp

include"auto.h"

void automobile::initialize(int in_mpg,float in fuel)

{

miles_per_gallon=in_mpg;

fuel_capacity=in_fuel;

)

int automobile::get_mpg() //提供一辆特定汽车每加仑公里数

{return miles per_gallon;}

float automobile::get_fuel() //提供油箱容积

{return fuel_capacity;}

float automobile::travel_distance()

{return (2) }

//car.h

ifndef CAR_H

define CAR_H

include"auto.h"

class car: (3)

{

int Total_doors;

public:

void initialize(int in_mpg,float in_fuel,int doors=4);

int doors(void);

};

endif

//car.cpp

include"car.h"

void car::initialize(int in_mpg,float in_fuel,int door)

{

Total_doors=door;

miles_per_galion=in_mpg;

fuel_capacity=in_fuel;

}

int car::doors(void)

{return Total doors;}

//Allauto.cpp

include

include"auto.h"

include"car.h"

int main()

{

car sedan;

sedan.initialize(24，20.0，4);

tout＜＜"The sedan can travel"＜＜ (4) ＜＜"miles.\n";

cout＜＜"The sedan has"＜＜ (5) ＜＜"doors.\n";

return 0;

}