下列事实中，物体运动状态没有发生改变的有（　　）

下列实验中，采用多次测量，其目的相同的是哪几组
①研究串联电路的电流特点    ②伏安法测定值电阻的阻值
③研究杠杆平衡条件          ④探究使用定滑轮的特点

有历史学家说：“中国的政治统一完成于秦始皇，而中国的文化统一完成于汉武帝。”请回答下列问题。

请用化学方程式表示下列变化：
（1）实验室用稀硫酸与锌粒反应制取氢气：____．
（2）向澄清石灰水中通人二氧化碳气体，石灰水变浑浊：____．

根据短文内容，选择正确答案。
A
    One day, Mr. and Mrs. White went shopping by car. They stopped their car near a store. They bought a lot of things and they wanted to put them in their car. But Mr. White couldn't open the door of the car. “Let's ask a policeman for help.” said Mrs. White. They asked a policeman to help them. The policeman was very friendly to help them open the door of the car. Just then a man came up and shouted, “What are you doing with my car?” Mr. and Mrs. White took a look at the number of the car. It was not their car.