This weekend is Tim's ____(nine) birthday, he asked his father to buy him ____new ipad. But this worries Tim's father much, can Tim use the ipad ____ (correct)? Will it be a good or bad present for the little boy?

    We all know that the computer is a great ____ (invent) in the world, it is very ____ (use) for people's life. It's interesting for people ____ watch movies or play games with ipads or computers. Now ____ (million) of people are using it every day.____it also may be bad to young people in some ways. Some children are very ____ (interest) in playing games or reading books, it may hurt ____ (they) eyes and make their study worse. All the parents hope their children can find better ways of using it.

    Tom has three good friends. They are Jack, David and Mark. They have different hobbies. Tom likes football. He often plays football on weekends. He thinks playing football can make him strong and healthy. He is in the school football team.

    Jack doesn't like sports. He likes listening to music. He likes Jay Chow best of all the singers. He has many of his CDs. He often listens to music on the way. He thinks music can let him feel happy.

    David doesn't like sports or music. He likes stamps. He collects stamps for five years. The stamps are very beautiful. He thinks collecting stamps can help him know more about the world.

    Mark likes taking photos. He has a few cameras. When he is free, he often takes out his camera and takes some pictures. Some of his pictures are in the newspapers. He thinks the world is beautiful. He wants to show the beauty of the world to others.

甲物质的密度为5g/cm³，乙物质密度为2g/cm³，各取一定质量混合后密度为3g/cm³．假设混合前后总体积保持不变，则所取甲、乙两种物质的质量之比是(    )