input: Write a function to classify student grades based on their scores.
  assert classify_grades([90, 85, 75, 60, 95, 50, 89])==['B', 'B', 'C', 'D', 'A', 'F', 'B']
  assert classify_grades([100, 80, 70, 60, 50])==['A', 'B', 'C', 'D', 'F']
  assert classify_grades([92, 88, 77, 66, 55])==['A', 'B', 'C', 'D', 'F']
output: def classify_grades(scores):
      grades = []
      for score in scores:
          if score >= 90:
              grades.append('A')
          elif score >= 80:
              grades.append('B')
          elif score >= 70:
              grades.append('C')
          elif score >= 60:
              grades.append('D')
          else:
              grades.append('F')
      return grades