据2015年央视“3•15”晚会曝光，山东省一些地区非法生产的调和汽油中含有甲缩醛（C₃H₈O₂），对汽车油路有破坏作用．下列说法不正确的是（　　）

下列代数式：2xy²，-$\frac{1}{2}$ab，$\frac{1}{4}$，2b=1，m，$\frac{a+b}{4}$，$\frac{a}{\pi }$中，单项式有（　　）

语法填空。

    I'm Lucy．This is a map of our school．____six buildings(楼房)in it．They are a dining hall，an art building，a sports hall，a science building，a classroom building and a library．Our classroom building is ____the science building．And it's ____ the library．In the middle of our school,____a big playground(操场)．We often play sports on it．Next to the playground is the art building．It's____the dining hall and the sports hall．Our school is big，and it's nice．