input: Write a function to respond to user input based on predefined questions and responses using random.choice.
  assert respond("What is your favorite movie?") in responses
  assert respond("What is your favorite color?") in responses
  assert respond("How are you?") == "I'm sorry, I don't understand."
output: import random
  
  responses = {
      "What is your favorite movie?": ["The Godfather", "Pulp Fiction", "Inception"],
      "What is your favorite color?": ["Blue", "Green", "Red"]
  }
  
  def respond(input_string):
      if input_string in responses:
          return random.choice(responses[input_string])
      else:
          return "I'm sorry, I don't understand."