根据短文理解选择正确答案。

                                                        Keeping a diary—a good way to improve English

     When I was at the age of thirteen, I began to keep a diary in English. At first it seemed difficult to me, but now it has begun to be easy. I know it's not possible to learn English well without more practice, I like reading, listening and speaking. After school I often go to the school library to read English stories. My English teacher always helps me translate some passages into Chinese. 

     I have little time to watch TV, or to play computer games. After I finish writing my diary, I always try to correct mistakes with the help of the dictionary. I think that keeping a diary is one of the best ways to improve our English study. 

按照题目要求作图．

（1）如图甲，做出以力臂L为力臂的动力F₁及物体所受重力示意图；

（2）以笔带线画出最省力的滑轮组绕法；

（3）在图所示电路的〇里填上适当的电表符号，使电阻R₁与R₂并联．

设函数$f\left(x\right)=\left(x^2-8x+c_1\right)\left(x^2-8x+c_2\right)\left(x^2-8x+c_3\right)\left(x^2-8x+c_4\right)$，集合$M=\left\{x|f\left(x\right)=0\right\}=\left\{x_1,x_2,\cdots ,x_7\right\}$$\subseteq N^{*}$，设$c_1\ge c_2\ge c_3\ge c_4$，则$c_1-c_4=$(    ）