下列代码中，执行 d1.work() 和 d2.work() 输出不同结果的主要原因是

class Device:
    def work(self):
        print("Device is working")

class Printer(Device):
    def work(self):
        print("Printer is printing")

class Scanner(Device):
    def work(self):
        print("Scanner is scanning")

if __name__ == '__main__':
    d1 = Printer()
    d2 = Scanner()
    d1.work()
    d2.work()