input: Write a python function to classify angles as zero, acute, right, obtuse, or straight.
  assert classify_angle(0) == "Zero angle"
  assert classify_angle(45) == "Acute angle"
  assert classify_angle(90) == "Right angle"
  assert classify_angle(135) == "Obtuse angle"
  assert classify_angle(180) == "Straight angle"
output: def classify_angle(angle):
      if angle == 0:
          return "Zero angle"
      elif 0 < angle < 90:
          return "Acute angle"
      elif angle == 90:
          return "Right angle"
      elif 90 < angle < 180:
          return "Obtuse angle"
      elif angle == 180:
          return "Straight angle"