A、The volunteers will edit it. B、The volunteers will delete it. C、The person posting it will reverse it. D、The person posting it

You’ve probably seen encyclopedias. Whether you’re settling an argument or researching a school project, these books can hold the answers. These days though, the world moves so fast, it’s hard for books that were written months or years ago to keep up. Thankfully we have a new kind of encyclopedia that’s online, free, built by thousands of people and changes every day. This household name is: Wikipedia.
    The idea that thousands of volunteers could create an online encyclopedia doesn’t sound possible, but thanks to new technology and specific policies, Wikipedia has become one of the top 5 sites on the Web. The site is run by a not-for-profit foundation with a goal to provide everyone on the planet access to the sum of all human knowledge. To see how it works, let’s get started with the "wiki" in Wikipedia.
    A wiki is a kind of website that allows users to make changes to any page. They simply click, edit, make the change, and then save the page to share it with the world. This basic concept allows volunteers to contribute information on any subject in Wikipedia. And because Wikipedia is a website, there is no limit to the number of topics it can cover. Being a wiki means that Wikipedia is always changing. When someone notable passes away, Wikipedia is updated. When news breaks, Wikipedia grows. It all works because tens of thousands of volunteers contribute, and also enforce rules to ensure Wikipedia remains a reliable source for factual information.
    These users, like any member, can see changes as they happen on each article. For example, if someone posts an advertisement on Wikipedia, which is forbidden, volunteers can easily reverse the change to maintain the article’s integrity. This means every change to Wikipedia is reviewed and must observe two big rules.
    The first is verifiability, which is necessary to ensure high quality. For this reason, Wikipedia articles must rely on information from published sources like books or newspapers—resources known for fact-checking. Requiring contributors to cite these resources in articles and quotations ensures Wikipedia articles are factual and high quality. If it’s not verified, it can’t be in Wikipedia.
    The second rule requires a neutral point of view. All Wikipedia materials must be presented fairly and without bias, just like any other encyclopedia. This means Wikipedia is not a place for contributors to share their own opinions.
    It’s these two rules and the volunteers who uphold them that make Wikipedia a reliable resource that grows each day. And you can be involved in the process of building a free encyclopedia—the largest encyclopedia in human history.
16. What can we learn about Wikipedia from the talk?
17. What can users do in a wiki?
18. What will probably happen when an advertisement is posted on Wikipedia?
19. What is the second rule of Wikipedia?