python3中 提示NameError: name 'grade' is not defined 新手的我，蒙了

score = int(input('Please Enter the score:'))
if score >= 90:
grade = 'A'
elif 60 =< score < 89:
grade = 'B'
elif score < 60:
grade = 'C'

print ('%d is %s' % (score,grade))