我被指派創建一只名為'Molly'age的狗1。我調用方法grow來增加年齡2并打印它。我知道如何讓莫莉跑，但我對如何增加她的年齡感到困惑，它一直在顯示None。任何幫助都會起作用，但最好能解釋一下。提前感謝您的幫助。class Dog():    """ a class representing a dog """    def __init__(self, name, age):        """ Initialize name and age attributes. """        self.name = name        self.age = age    def run(self):        """ Simulate a dog running. """        print(self.name.title() + " is running ...")    def grow(self, year):        """ Update age by adding year. """        self.age += yearmolly = Dog("Molly", 1)print(molly.name + " is " + str(molly.grow(2)))molly.run()這個的輸出是Molly is NoneMolly is running ...