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问题 选择题

关于功和能，下列说法正确的是（　　）

A．合外力对物体做正功，机械能一定增加

B．一对相互作用力可能都做正功

C．系统内力做的功对系统的总动能无影响

D．-个物体以大小为2m/s2的加速度竖直向上做匀减速运动，其机械能一定增加

答案

A、合外力做正功，根据动能定理，动能一定增加，故A错误；

B、一对相互作用力方向相反，但作用在不同的两个物体上，由于两个物体的位移可能相反，并都与各自的力同向，故可能都做做功，故B正确；

C、对物体系统运用动能定理时，内力和外力做的功之和等于系统总的动能增加量，故C错误；

D、-个物体以大小为2m/s2的加速度竖直向上做匀减速运动，除重力外，一定有其他力做功，故机械能不守恒，故D错误；

故选B．

单项选择题

Are you dreaming of taking a trip on your own Here is some advice on how to make your once-in-a-lifetime journey and how to stay safe when you get there.
Before you begin to look at places, activities and plane tickets, you must set a budget. When you search through the websites about travelling, it can be so easy to add another week here or there, or choose an extra activity. Setting a budget can avoid creating a huge bill, which starts very small.
With so many countries on this planet it is difficult to decide where to go. By narrowing down your search to voluntary work or a relaxing holiday, you can save a lot of time looking through websites.
After deciding where to go, you need to research the country. Find out when each season is. It would be disappointing if you were taking part in an outdoor-based activity and it rained most days. Look into medical information, as you may need certain medicine for certain countries. Above all, make sure you can afford to go to this country and take part in the activities you have dreamt about. Don’t raise your hopes too high before you know for certain it is within your budget.
By carrying a credit (信用) card with you at all times it is possible to stop yourself getting lost. Mobile phones are very important as well. However, do not depend completely on them, since electronic machines can break without warning. Therefore, always carry a list of emergency (紧急情况) phone numbers, names and addresses. Finally, learn a little of the language. Do not expect that the people there speak your own language. After all, you are in their country.

From the text, we can know that ______.

A．travelling in a foreign country costs a lot
B．deciding on where to go is an easy job
C．a volunteer can help you choose where to go
D．you need to know what the weather of the county you’ll travel is like
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判断题

在我国，基金托管人只能由依法设立并取得基金托管资格的商业银行和信托公司担任。( )
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