这段程序会输出什么？

#include <iostream>

class Sender {
public:
    virtual void send(int priority = 1) const {
        std::cout << "base:" << priority << '\n';
    }
    virtual ~Sender() = default;
};

class EmailSender final : public Sender {
public:
    void send(int priority = 2) const override {
        std::cout << "email:" << priority << '\n';
    }
};

int main() {
    EmailSender email;
    Sender& sender = email;
    sender.send();
    email.send();
}