class Student(object):    def __init__(self, name, score):        self.name = name        self.score = score    def get_grade(self):        if self.score < 0 or self.score > 100:            raise ValueError('invalid score value:0-100')        if self.score >= 80:            return 'A'        if self.score >= 60:            return 'B'        return 'C'