import unittestclass Student(object):    def __init__(self, name, score):        self.name = name        self.score = score    def get_grade(self):        if self.score>=80 and self.score<=100:            return 'A'        elif self.score >= 60 and self.score<80:            return 'B'        elif self.score>=0 and self.score<60:            return 'C'        else:            raise ValueError('invalid grades')