下列有关电磁波的说法，正确的是   （    ）

平面直角坐标系中，点集$M=\left\{\right(x,y\left)\right|\left\{\begin{array}{c}x=\sin \alpha +\cos \beta \\ y=\cos \alpha -\sin \beta \end{array}\right.$$\alpha ,\beta \in R\}$，则点集M所覆盖的平面图形的面积为（   ）

    In America, the school day usually starts at 9a.m. and 1at about 4 p.m. The school subjects are different from 2. In China, we always have Chinese, maths, English, P.E. and so on. But in America, students have more lessons, such as 3 and driving lessons. Their school life isn't just about study. They have 4things to do than the Chinese students do. There 5 many clubs in American schools, too. If the students are6in sports, they can join different sports clubs. From 2:30 p.m.74 p.m., they do different things in different clubs.

    Maybe you think English is very popular in the world,8 American students don't need to learn any 9 language. But that's not true, American children from the age of seven 10 learn a foreign language. For example, they learn French or Chinese.