input: Write a python function to classify numbers as positive, negative, or zero.
  assert classify_numbers([1, -2, 3, 0, -5, 6]) == ['positive', 'negative', 'positive', 'zero', 'negative', 'positive']
  assert classify_numbers([0, 0, 0]) == ['zero', 'zero', 'zero']
  assert classify_numbers([-1, 1, -1, 1]) == ['negative', 'positive', 'negative', 'positive']
output: def classify_numbers(numbers):
      result = []
      for num in numbers:
          if num > 0:
              result.append('positive')
          elif num < 0:
              result.append('negative')
          else:
              result.append('zero')
      return result