学习科学需掌握一定的科学方法，明确实验原理及并能对结论进行科学推理．

（1）我们在科学实验时，为了减小误差或寻找普遍规律，经常需要进行反复多次实验．

①“测量物体的长度”时，多次测量；

②“研究杠杆的平衡”时，改变动力（臂）和阻力（臂），多次测量；

③“研究串、并联电路的电流特点”时，换用不同定值电阻，多次测量；

④“用电压表和电流表测导体的电阻”时，多次测量电阻两端电压和通过电阻的电流值；

上述实验中寻找普遍规律的是​ ____

    Hello! Nice to meet you. Now let me introduce myself (自我介绍) to you. My name is Mary. I'm 12 years old. I'm from Canada and I speak English. I'm a tall girl. I have long brown hair and big blue eyes. I like music a lot. I often sing songs with my friends in my free time. I have a happy family. There are four people in my family. They are my mom, my dad, my sister and me. I like China very much and I want to visit Beijing, but I can't speak Chinese at all.

    I hope (希望) to be your friend, then we can help each other with English and Chinese.

根据材料内容，选择最佳答案。

G20峰会期间，******与多国领导举行会谈，就共同关心的重大国际地区问题，深入交流，交换意见，并达成一系列共识。这表明（    ）

There is a ten-minute break between the classes.

下列关于治理黄土高原和云贵高原的生态治理措施，正确的是（　　）