input: Write a python function to extract book titles from the provided HTML.
  assert extract_titles(html) == ["Python Programming", "Advanced Python", "PHP Programming"]
  assert extract_titles("<html><body><table><tr><td>123</td><td>Book 1</td></tr></table></body></html>") == ["Book 1"]
  assert extract_titles("<html><body><h1>Empty</h1></body></html>") == []
output: def extract_titles(html):
      from bs4 import BeautifulSoup
  
      soup = BeautifulSoup(html, 'html.parser')
      titles = [td.get_text() for td in soup.find_all('td') if td.find_previous_sibling()]
      
      return titles