input: Write a python function to replace all 5-letter words in a sentence with '_____'.
  assert replace_five_letter_words('The quick brown fox jumped over the lazy dog') == 'The _____ brown _____ jumped over the _____ _____'
  assert replace_five_letter_words('Hello world, this is a test') == 'Hello world, this is a _____'
  assert replace_five_letter_words('Python code can be very interesting') == 'Python code can be very _____'
output: def replace_five_letter_words(sentence):
      words = sentence.split()
      for i in range(len(words)):
          if len(words[i]) == 5:
              words[i] = '_____'
      return ' '.join(words)