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问题 选择题

一个物体在7个恒力的作用下处于平衡状态，现撤去其中两个力，其它力大小和方向均不变．则关于物体的运动下列说法正确的是（　　）

A．可能做圆周运动

B．一定做匀变速曲线运动

C．可能处于静止状态

D．一定做匀变速直线运动

答案

物体在几个力的作用下处于平衡状态，若撤去其中某两个力而其余力的性质（大小、方向、作用点）不变，根据平衡条件，其余力的合力与撤去的两个力的合力等值、反向、共线，合力是恒力，若撤去的两个力是一对平衡力，则物体仍受力平衡．

A、圆周运动需要有力去提供向心力，所以合力一定是个变力，根据分析可知，剩余的力的合力不可能是变力，故A错误；

B、若撤去的两个力是一对平衡力，则物体仍受力平衡，物体仍做匀速直线运动或处于平衡状态，故B错误，C正确；

D、当剩余的力的合力与速度方向不在同一直线上时，物体做匀变速曲线运动；故D错误；

故选：C．

完形填空

根据下面短文内容，在短文的空格处填上一个恰当的词，使短文完整、通顺。

People usually check whether their keys, wallets and phones are with them___小题1:___ going out. But many people in Chain now have one more important thing to take, a face mask(面罩). They hope that it __小题2:____ protect them from polluted air.

Since January, many cities in China have seen a lot of hazy(雾蒙蒙的)days. In Beijing, ____小题3:___five days in January were free of smog(雾霾). People in Xi’an only saw a blue sky on one day in February. Smog is dirty air that ___小题4:___like fog and smoke.

An official for the Ministry of Environmental Protection told Beijing News that PM2.5 pollution is the main problem with air pollution. PM2.5 refers to pollution with small particles(颗粒) of 2.5 micrometers(微米) or less in diameter(直径).

According to the Chinese Academy of Sciences, ___小题5:_____cars and burning coal(煤) are two major factors(原因) for PM2,5.

Poor air is___小题6:___ for people’s health, especially for children and the elderly. A report from Beijing Children’s Hospital shows that from Jan 5 to 11, half of the patients in the internal medicine department(内科门诊) are there for respiratory(呼吸) problems.

Air problems and environmental protection have become a concern(关切的事) for the whole nation. Premier Wen Jiabao___小题7:____ his last Government Work Report on March 5. the report said that the government should work hard to prevent pollution and give people a ____小题8:____living environment.

It calls on people to make full use of energy and resources and cut down on energy use.
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单项选择题

我国现行增值税征收对小规划纳税人采用( )的征收率。

A．17%
B．13%
C．7%
D．6%
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