input: Write a python function to extract user reviews from a webpage using Selenium.
  assert len(extract_reviews("http://example.com")) > 0
  assert isinstance(extract_reviews("http://example.com")[0], str)
  assert "excellent" in extract_reviews("http://example.com")[0].lower()
output: def extract_reviews(url):
      from selenium import webdriver
      from selenium.webdriver.common.by import By
      from selenium.webdriver.chrome.service import Service
      from selenium.webdriver.chrome.options import Options
      from webdriver_manager.chrome import ChromeDriverManager
      
      # Setting up Chrome options
      chrome_options = Options()
      chrome_options.add_argument("--headless")  # Ensure GUI is off
      chrome_options.add_argument("--no-sandbox")
      chrome_options.add_argument("--disable-dev-shm-usage")
      
      # Set up the WebDriver
      driver = webdriver.Chrome(service=Service(ChromeDriverManager().install()), options=chrome_options)
      
      try:
          # Open the webpage
          driver.get(url)
          
          # Extract reviews - this will vary based on the webpage structure
          reviews_elements = driver.find_elements(By.CLASS_NAME, 'review-class')  # Adjust the class name accordingly
          reviews = [element.text for element in reviews_elements]
          
      finally:
          driver.quit()
      
      return reviews