下列代码中，d1->work(); 和 d2->work(); 输出不同结果的主要原因是（ ）。

class Device {
public:
    virtual void work() {
        cout << "Device is working" << endl;
    }
    virtual ~Device() {}
};

class Printer : public Device {
public:
    void work() override {
        cout << "Printer is printing" << endl;
    }
};

class Scanner : public Device {
public:
    void work() override {
        cout << "Scanner is scanning" << endl;
    }
};

int main() {
    Device* d1 = new Printer();
    Device* d2 = new Scanner();

    d1->work();
    d2->work();

    delete d1;
    delete d2;
    return 0;
}
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