input: Write a function to extract all the words from a given string that start with a specific letter. The function should take a string and a character as input and return a list of words that start with the specified character.
  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', 'the', 'program']
  assert extract_words('Python is fun', 'P')==['Python']
  assert extract_words('Hello World', 'h')==[]
  assert extract_words('An apple a day keeps the doctor away', 'a')==['apple', 'a', 'away']
  assert extract_words('', 's')==[]
output: def extract_words(word, letter):
      words_list = []
      for w in word.split():
          if w[0] == letter:
              words_list.append(w)
      return words_list