The answer is flash mob. They called it flash mob when a large number of people in twitter gather in group for a centralized face to face meeting. In flash mob, people are gathering to be able to create a community that is helping each other or even just talking to each other.
A navigation bar (or navigation system) is a section of a graphical user interface intended to aid visitors in accessing information. Navigation bars are implemented in file browsers, web browsers and as a design element of some web sites.
Answer:
A: used by ISP's to filter out email SPAM
C: a way to help an individual focus on best choices when deciding what to watch or buy.
Explanation:
Collaborative filtering uses a community-based approach to filter spam. It works by collecting numerous email users from around the world. By doing this, it becomes possible for users to flag emails that are spam and those that are legitimate.
Also Collaborative Filtering is one of the most efficient techniques for building a system that can help a user when it comes to recommending best choices based on information from a large number of users.