A、They have done better than conventional cars. B、They have caused several severe crashes. C、They have posed a threat to other d

One of Google’s self-driving cars crashed into a bus in California last month. There were no injuries.
    It is not the first time one of Google’s famed self-driving cars has been involved in a crash, but it may be the first time it has caused one.
    On February 14th the self-driving car, traveling at 2mph (3km/h), pulled out in front of a public bus going 15mph (24km/h).
    (1) The man in the Google vehicle reported that he assumed the bus would slow down to let the car out, and so he did not switch to the manual mode.
    In a statement, Google said: "We clearly bear some responsibility, because if our car hadn’t moved, there wouldn’t have been a crash."
    That’s said, our test driver believed the bus was going to slow or stop to allow us to merge into the traffic, and that there would be sufficient space to do that.
    (2) The company’s self-driving cars have done well over a million miles across various states in the US, and until now have only reported minor accidents.
Questions 1 and 2 are based on the news report you have just heard.
1. According to Google, what was the cause of the accident?
2. How have Google’s self-driving cars performed so far?