The exam comes in July. When the exam finishes, the summer vacation begins. Boys and girls have two months to rest. The summer vacation is the best part of the year for most children. The weather is usually good. They can swim, go to summer camp or visit other places with their parents.

    We all know that the beaches are good places to relax. Some children are lucky to live near the sea. They can enjoy the sea at any time. But for the children who live far from the sea, they only go to the beaches for one or two weeks with their parents.

    Why do children like spending their summer vacation on the beaches? It is because they like the sand, the sun, the cool wind and the salt water. There are a lot of new things to see, nice things to eat and exciting things to do.

有170个梨，平均放在9个盘里，每个盘里放多少个？还剩几个？验算一下。

如果用³²P和³⁵S分别标记噬菌体的DNA和蛋白质外壳，当它侵染到细菌体内后，经多次复制，所释放出来的子代噬菌体       　　　　（　　）

下列三张图所揭示出的我国古代商业活动中心发展的趋势是（    ）