如图所示的电路，电源电压保持不变，滑动变阻器的滑片P置于中点位置．闭合开关后，电压表示数为U，电流表A₁、A₂示数分别为I₁、I₂，且灯丝电阻不变，下列分析正确的是（　　）

在腔肠动物的身体结构中，不具细胞结构的一层结构是（    ）

根据短文内容，判断正确还是错误。

     Lynn has many books and toys. She has English books, Chinese books, word books and comic books. The English books and word books are thick. The Chinese books and comic books ard thin. She likes reading English books and word books. She has a boy bear, a toy dog and a toy cat. Her toy bear is hard. Her toy dog and toy cat are soft. She likes them very much.

下列各句中，划线的成语使用正确的一项是（     ）