input: Write a function to extract all the words in a given sentence that start with a specific letter. The function should accept two parameters: the sentence as a string and the specific letter as a string. The function should return a list of words that match the criteria.
  
  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
  
  assert extract_words("The sun rises in the east", "s") == ["sun"]
  assert extract_words("She sells seashells by the seashore", "s") == ["She", "sells", "seashells", "seashore"]
  assert extract_words("This is a test sentence", "a") == ["a"]
  assert extract_words("All these apples are amazing", "A") == ["All", "apples", "are", "amazing"]
  assert extract_words("", "z") == []
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