选择题   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______。