class Student(object):

def __init__(self,name,score):
    self.name = name
    self.score = score

def get_grade(self):
    if (0 <= self.score <= 100):
        if(self.score >= 80):
            return 'A'
        if(self.score >= 60):
            return 'B'
        return 'C'
    else:
        raise ValueError('invalid score：%s' %self.score)