input: Write a python function to simulate a car race and determine the winner.
  assert race() in ["Car A wins!", "Car B wins!"]
  assert isinstance(race(), str)
output: def race():
      import random
      car_a_speed = random.randint(100, 200)
      car_b_speed = random.randint(100, 200)
      if car_a_speed > car_b_speed:
          return "Car A wins!"
      elif car_b_speed > car_a_speed:
          return "Car B wins!"
      else:
          return "It's a tie!"