input: Write a function to create a virtual assistant using Python and Dialogflow to answer a basic set of questions. The function should take user input and return the assistant's response. If the user inputs 'exit', the program should terminate.
  assert virtual_assistant("What is the capital of France?")=="The capital of France is Paris."
  assert virtual_assistant("How tall is the Eiffel Tower?")=="The Eiffel Tower is approximately 324 meters tall."
  assert virtual_assistant("exit")==None
output: def virtual_assistant(user_input):
      if user_input == "exit":
          return None
      responses = {
          "What is the capital of France?": "The capital of France is Paris.",
          "How tall is the Eiffel Tower?": "The Eiffel Tower is approximately 324 meters tall."
      }
      return responses.get(user_input, "I don't know the answer to that question.")