input: Write a python function to determine the grade based on the score.
  assert determine_grade(95) == 'A'
  assert determine_grade(85) == 'B'
  assert determine_grade(75) == 'C'
  assert determine_grade(65) == 'D'
  assert determine_grade(55) == 'F'
output: def determine_grade(score):
      if score >= 90:
          return 'A'
      elif score >= 80:
          return 'B'
      elif score >= 70:
          return 'C'
      elif score >= 60:
          return 'D'
      else:
          return 'F'