根据短文内容，选择正确答案。
Hello, friends!
    My name is Zhou Dan. I'm a Chinese girl. I'm twelve. I'm from Zhengzhou, China. Do you know Zhengzhou? It's in the middle of China. I'm in No. 8 Middle School. There are about 900 students and 50 teachers in our school. It is a good school in Zhengzhou: I'm in Class 2 Grade 7. There are 49 students in our class: 29 girls and 20 boys. My English teacher is Miss Zhao Dan. Our names are almost the same, right? She is nice to me. We're good friends. I like English, and I like music, too. I want to find a pen-friend. Please write to me.
                                                              Yours,
                                                            Zhou Dan

7434－456＋974=（   ）

在△ABC中，sinA：sinB：sinC= $\sqrt{3}$ ：4： $\sqrt{31}$ ，则角C的大小为（   ）

“军国大计，罔不总揽。自雍乾后百八十年……盖隐然执政之府也。”这里的“执政之府”是(   )

关于“学习使用显微镜”和“科学探究的基本过程”的叙述，正确的是（    ）