input: Write a function to extract all the words starting with a specific letter from a given string. The function should return a list of words that start with the specified letter.
  
  def extract_words(word, letter):
      words_list = []
      for w in word.split():
          if w[0] == letter:
              words_list.append(w)
      return words_list
  
  assert extract_words('This is a sample sentence to test the program', 's') == ['sample', 'sentence', 'test']
  assert extract_words('Apple is a fruit', 'a') == []
  assert extract_words('snake squirrel sea', 's') == ['snake', 'squirrel', 'sea']
  assert extract_words('', 'a') == []
  assert extract_words('Awesome animals', 'A') == ['Awesome']
output: def extract_words(word, letter):
      words_list = []
      for w in word.split():
          if w[0] == letter:
              words_list.append(w)
      return words_list