若关于$x,y$的方程组$\left\{\begin{array}{c}ax+by=1\\ x^2+y^2=1\end{array}\right.$有实数解，则实数$a,b$满足（   ）

因人之力而敝之，不仁；____，____；以乱易整，不武。（《左传》）

根据短文理解，完成各题任务。
C
    Sally is an English girl. She is my English teacher, Mrs. Brown's daughter. She is only five years old. (A)She can't go to school. She likes playing ping-pong.
    Her favorite ping-pong player is Zhang Yining. Sally is in the ping-pong club. She goes to the ping-pong club every day. She plays ping-pong for two hours there.
    Sally has a sister. Her name is Maria. She is in our class. She likes running. She runs with her mother every morning. (B)她最喜欢的赛跑明星是刘翔. Maria says she wants to be a running star like Liu Xiang.

小明在家探究鸡蛋受到的浮力大小与哪些因素有关，如图所示，鸡蛋浸没在水中时，所受到的浮力为____ 　N；如果直接将该鸡蛋投入水中，该鸡蛋将　____ 　（填“上浮”“悬浮”或“下沉”）；鸡蛋的密度是　____ 　kg/m³。如果向水中加盐并轻轻搅拌，最终鸡蛋悬浮在盐水中，则此时盐水的密度　____ 　（填“大于”“小于”或“等于”）鸡蛋的密度。（g=10N/kg）

国家统一是中国历史发展的主流。下列政权中没有实现对国家统一的是（    ）