单选题 有如下程序：
#include＜iostream＞
using namespace std;
class Boat;
class Car{
public:
Car(int i):weight(i){}
friend int Total(const Car &c.const Boat&b); //①
private:
int weight;
};
class Boat{
public:
Boat(int i):weight(i){}
friend int Total(const Car &c, const Boat &b);
private:
int weight;
};
int Total(const Car &c, const Boat &b){ //②
return c.weight+b.weight;
}
int main(){
Car c(10);
Boat b(8);
cout＜＜"The total weight is"＜＜Total(c, b)＜＜endl; //③
return 0;
}
下列关于程序的描述中，正确的是______。