input: Write a python function to convert a score into a grade.
  assert get_grade(85) == 'B'
  assert get_grade(92) == 'A'
  assert get_grade(73) == 'C'
  assert get_grade(60) == 'D'
  assert get_grade(59) == None
output: def get_grade(score):
      if 90 <= score <= 100:
          return 'A'
      elif 80 <= score < 90:
          return 'B'
      elif 70 <= score < 80:
          return 'C'
      elif 60 <= score < 70:
          return 'D'
      elif 0 <= score < 60:
          return None
      else:
          return None