昼夜更替产生的原因是（    ）

已知$\left\{\begin{array}{c}x=2\\ y=1\end{array}\right.$是二元一次方程组$\left\{\begin{array}{c}mx+ny=8\\ nx-my=1\end{array}\right.$的解，则2m-n的算术平方根为（   ）

阅读理解，判断文后句子的正误。

    My name is Amy. Tomorrow is Saturday. We have no classes. My parents aren't going to work, but we are going to be very busy tomorrow. My mother is going to buy something for next week. My father is going to visit my aunt and uncle. I'm going to go to the bookstore by bus. Then I'm going to buy a dictionary. Tomorrow evening we are going to visit my grandparents and have a big dinner. After that we are going to go to the cinema. I think we are going to have a nice weekend.