用方框所给出选项完成对话。

Polly: Dad, I want to get a job.

Father : ____ 

Polly: I need money. When I go out with my friends, I never have enough money.

Father: Polly, you know most families here are richer than we are.

Polly: ____  Then, I won't have to ask you for money.

Father: You have a lot of homework everyday. How can you work and study at the same time?

Polly: ____

Father: I have an idea. Once a week, you can come to my shop and help out a bit. You can clean the floors and the windows－that sort of thing. I’ll pay you for each hour you work. How does that sound?

Polly: ____  But I don't want to clean any toilets(卫生间).

Father: ____  It's a deal.

我们的银河系的恒星中大约四分之一是双星．某双星由质量不等的星体S₁和S₂构成，两星在相互之间的万有引力作用下绕两者连线上某一定点C做匀速圆周运动．由天文观察测得其运动周期为T，S₁到C点的距离为r₁，S₁和S₂的距离为r，已知引力常量为G．由此可求出S₂的质量为（   ）

材料一：某校历史兴趣小组根据书中“近代中国社会的发展轨迹像个元宝形，开始是下降，降到谷底，然后上升，升出一片光明……”的描述，绘制了下图。

材料二：“英国军舰公然炮击进攻武汉的北伐军……各帝国主义集结在上海……随时准备进攻北伐军。”

51÷9=____