input: Write a python function to recognize voice commands and respond accordingly.
  assert recognize_voice_command('hello')=='Hello! How can I help you?'
  assert recognize_voice_command('hi there')=='Hello! How can I help you?'
  assert recognize_voice_command('goodbye')==None
output: def recognize_voice_command(command):
      responses = {
          'hello': 'Hello! How can I help you?',
          'hi there': 'Hello! How can I help you?'
      }
      return responses.get(command)