A、Because people don’t know each other well any more. B、Because people want to keep their own secrets. C、Because a lot of bad gu

Moderator:      
    Today’s speaker is Danny Hillis, an inventor, scientist, author and engineer. While completing his doctorate at MIT, he pioneered the concept of parallel computers that is now the basis for most supercomputers. Throughout his career, Hillis has worked at places like Disney and now Applied Minds, always looking for the next fascinating problem. Now, let’s welcome Mr. Hillis.   
    Mr. Hillis:      
    Thank you. Ladies and gentlemen, please look at this book that I have in my hand. It is a book that listed everybody who had an email address in 1982.There were only two other Dannys on the Internet then. I knew them both. We didn’t all know each other, but we all kind of trusted each other, and there was a real sense that we could depend on each other to do things.
    The attitude of only taking what you need was really what everybody had on the network in those days, and in fact, it wasn’t just the people on the network, but it was actually kind of built into the protocols of the Internet itself. It was actually interesting that such a communist principle was the basis of a system developed by the Defense Department, but it obviously worked really well. In fact, it was so successful that there’s no way that these days you could make a book like this. My rough calculation is it would be about 25 miles thick. But you couldn’t do it. We don’t know the names of all the people with Internet. Because there’s a lot of bad guys on the Internet these days, and we have to deal with that by making walled communities, secure subnetworks, VPNs, etc.
    The fact is, people are mostly focused on defending the computers on the Internet, and there’s been surprisingly little attention to defending the Internet itself as a communications medium. And I think we probably do need to pay more attention to that, because it’s actually kind of fragile. So we’ve built this system where we understand all the parts of it, but we’re using it in a very, very different way than we expected, and it’s gotten a very, very different scale than it was designed for. And in fact, nobody really exactly understands all the things that being used for right now.  
    And the problem with it is, I think we are setting ourselves up for a kind of disaster like the disaster we had in the financial system. So what we need is a plan B. There is no plan B right now. There’s no clear backup system that we’ve very carefully kept to be independent of the Internet, made out of completely different sets of building blocks. I think this is actually the focus of the conference, and I’m happy to get a chance to tell you about it.      
    Thank you very much.
    Questions 16 to 19 are based on the recording you have just heard.
    16.What achievement did Danny Hillis make while he was at MIT?
    17.What does Danny show the audience at the beginning of his talk?
    18.Why do we have to make walled communities?
    19.What is the suggestion that Danny puts forward at the end of his talk?