A、In their appearance. B、In the form of displaying. C、In the visiting rules. D、In the opening time. B推断题。短文开头提到，博物馆已经发生了变化，它们已经不

Museums have changed. They are no longer places that one "should" go but now they are places to enjoy. At a science museum in Canada, you can feel your hair stand on end as harmless electricity passes through your body. At the Children’s Museum in New York, you can play an African drum. There are no "Do Not Touch" signs in some other museums in the USA. More and more museum directors have realized that people learn best when they can become part of what they are seeing. In many science museums, the visitors are encouraged to touch, listen, operate and experiment so as to discover scientific rules for themselves. The purpose is not only to provide fun, but also help people feel at home in the world of science. If people don’t understand science, they will be afraid of it; and if they are afraid of science, they will not make the best use of it.
    One cause of all these changes is the increase in wealth and spare time. Another cause is the growing number of young people in the population. Many of them are college students or college graduates. They see things in a new and different way. They want art that they can take part in. The same is true in science and history.
    The old museums have been changing and the government is encouraging the building of new, modern museums.[31]In the United States and Canada, there are more than 6,000 museums, almost twice as many as there were 25 years ago.
29. In what aspect have the museums changed, according to the passage?
30. What is one of the causes of the changes in museums?
31. How many museums were there 25 years ago in the United States and Canada?