input: Write a python function to respond to greetings, farewells, and unknown sentences.
  assert respond("hello") in GREETING_RESPONSES
  assert respond("bye") in FAREWELL_RESPONSES
  assert respond("what's happening?") in NOMATCH_RESPONSES
output: def respond(input_text):
      GREETING_RESPONSES = ["Hi there!", "Hello!", "Hey!"]
      FAREWELL_RESPONSES = ["Goodbye!", "See you later!", "Bye!"]
      NOMATCH_RESPONSES = ["I'm not sure how to respond to that.", "Could you rephrase that?", "I don't understand."]
  
      if input_text.lower() in ["hello", "hi", "hey"]:
          return GREETING_RESPONSES[0]
      elif input_text.lower() in ["bye", "goodbye", "see you"]:
          return FAREWELL_RESPONSES[0]
      else:
          return NOMATCH_RESPONSES[0]