下面哪种物质会浮在水上（       ）

    Mr. Lin has a store. It's next to No. 5 Middle School. It sells maps, books, pens, pencils, clothes and some other things. The things in his store are not expensive. Boys and girls like to go to his store. They like Mr. Lin because he is very nice. Mr. Lin likes the boys and girls, too. Mr. Lin has a “lost and found” box in his store. Look! A baseball, a ruler and some books are in the box. This box helps some boys and girls find their lost things. A sofa is in Mr. Lin's store, too. Boys and girls can sit there and read books after school. They can ask Mr. Lin questions about these books and he often helps them to find the answers.

A____；

B____ .