input: Write a function to extract all the words starting with a specific letter from a given sentence. The function should take in two parameters: the sentence and the specific letter, and return a list of words that start with the given letter.
  
  def extract_words(sentence, letter):
      words_list = []
      for word in sentence.split():
          if word[0] == letter:
              words_list.append(word)
      return words_list
  
  assert extract_words('This is a sample sentence to test the program', 's') == ['sample', 'sentence', 'test']
  assert extract_words('Python programming is fun', 'p') == ['Python', 'programming']
  assert extract_words('All these apples are red', 'a') == ['All', 'apples', 'are']
  assert extract_words('What a wonderful day', 'e') == []
  assert extract_words('', 's') == []
output: def extract_words(sentence, letter):
      words_list = []
      for word in sentence.split():
          if word[0].lower() == letter.lower():
              words_list.append(word)
      return words_list