阅读理解

    Tom,Bill and Dave lost their backpacks．They are at the lost and found office．The backpacks are the same(相同的),but the things in them are different．Can you help them find the right(正确的)backpack?

    Tom：I have a  maths book and a pencil case in my backpack．There are three pencils,a pen and an eraser in the pencil case．

    Bill：I have a Chinese dictionary,a maths book and two notebooks in my backpack．

    Dave：There are two CDs,three picture books in my backpack．My English book is also in it．

函数$f\left(x\right)=\tan \frac{x}{2}$的最小正周期为____ 

小强想写关于北宋内容的论文，下面哪个论文题目不能用（    ）

下列各项没有语病的一句是（    ）