input: Write a python function to classify emails as spam or not spam based on certain keywords.
  assert classify_email("Congratulations! You have won free cash. Click here to claim.") == "This email is classified as SPAM."
  assert classify_email("Hi there, can we schedule a meeting for tomorrow?") == "This email is classified as NOT SPAM."
  assert classify_email("Amazing discounts available on all products. Limited time offer!") == "This email is classified as SPAM."
output: def classify_email(email):
      spam_keywords = ["free", "win", "cash", "click", "offer", "discount", "limited time"]
      for keyword in spam_keywords:
          if keyword in email.lower():
              return "This email is classified as SPAM."
      return "This email is classified as NOT SPAM."