①锌为正极，石墨为负极②锌为负极，石墨为正极③工作时，电子由石墨极经过外电路流向锌极④长时间连续使用时，内装糊状物可能流出腐蚀用电器．

A

    Look! This is a picture of my teacher's family. This is Mr. Smith.  He  is my English  teacher. His wife is a doctor. She is nice. They have a son and a daughter. The boy's  name is Sam and the   girl's name is Kate. They are students. They are in the same school. They go to school on their bikes. Look! The son is like his mother. The daughter is like her father. Now they are all in China and can speak Chinese. They love China.

（1）解不等式：2x－1＜$\frac{x+2}{3}$；（2）解方程组：$\left\{\begin{array}{c}x=y+1\\ 2x+y=8\end{array}\right.$.

由图（甲）所示是验证力的平行四边形定则的实验装置，实验中，需要将橡皮筋的一端固定在水平木板上，另一端系上两根细绳（细绳的末端都有绳套），实验中需用两个____分别勾住绳套，并互成角度地拉橡

皮筋．某同学认为在此过程中必须注意以下几项：

A．两根细绳尽可能等长

B．橡皮条应与两绳夹角的平分线在同一直线上

C．在使用弹簧秤时要注意使弹簧秤与木板平面平行

其中正确的是____．（填入相应的字母）；

两位同学在实验时得到的实验结果如图乙所示，其中有一位同学操作有错．则乙图中____的实验结果与事实不符．

如图，一个质量为m的物体受到三个共点力F₁、F₂、F₃的作用，则物体所受的合力是（　　）