已知θ是第四象限角，则 $\sqrt{si{n}^2\theta -si{n}^4\theta }$ 可化简为（   ）

读我国各类地形面积比例示意图，回答下列问题。

看图写数。

                

      ____     ____         ____      ____ 

____的数叫做正数，____的数是负数，____既不是正数也不是负数．

    Mr. Brown is a teacher. He is from New York, America. He would like（想）to work in China because he likes our country. He thinks Chinese people are very friendly and he likes to live in China. He is happy to work in our school. He likes the school life here. He teaches English. There are 50 students in his class. Mr. Brown is kind. He and his students are good friends, too.
    Now some of Mr. Brown's students play sports on the playground（操场）. Bill and Mike play tennis. Tom and John play ping-pong. The other students play basketball. Mr. Brown is happy to play basketball with them. They have a good time.
阅读上面的短文，根据短文内容，选择最佳选项。