下面代码在 main() 中有一行会导致错误，请找出来

class Student:
    def __init__(self, n, s):
        self.__name = n
        self.__score = s

    def get_name(self):
        return self.__name

    def set_score(self, s):
        self.__score = s

if __name__ == '__main__':
    stu = Student("Tom", 85)
    print(stu.get_name())       # ①
    stu.set_score(90)           # ②
    print(stu.__score)          # ③
    print(stu.get_name())       # ④