input: Write a python function to respond to a message. 
  assert respond("How are you?") in ["I don't know :(", "You tell me!"]
  assert respond("What's up?") in ["I don't know :(", "You tell me!"]
  assert respond("Hello") in ["Hi there!", "Hello!", "Nice to meet you!"]
  assert respond("Greetings") in ["Hi there!", "Hello!", "Nice to meet you!"]
output: def respond(message):
      if message in ["How are you?", "What's up?"]:
          return "I don't know :("
      elif message in ["Hello", "Greetings"]:
          return "Hi there!"