input: Write a function to extract the title of a web page using BeautifulSoup.
  assert extract_title("<html><head><title>Test Title</title></head><body></body></html>") == "Test Title"
  assert extract_title("<html><head><title>Another Title</title></head><body></body></html>") == "Another Title"
  assert extract_title("<html><head><title></title></head><body></body></html>") == ""
output: def extract_title(html_content):
      from bs4 import BeautifulSoup
      soup = BeautifulSoup(html_content, 'html.parser')
      title_tag = soup.title
      return title_tag.string if title_tag else ""