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问题 选择题

一架飞机水平向右匀速飞行，从飞机上每隔1s释放一个铁球，先后共释放了四个，如果不计空气阻力，当地面上的人看到第四个球刚释放后，这四个球在空中位置关系是（　　）

A．

B．

C．

D．

答案

不计空气阻力，以地面为参考系，每个铁球都做平抛运动，水平方向做匀速直线运动，所以在水平方向上，四个铁球的速度总是与飞机速度相同的，水平位移相同，故没有位移差，看起来在一条竖直线上；竖直方向做自由落体运动，最先释放的铁球间的距离大些，相当于同一个铁球做自由落体运动在不同时刻的位置．

故选C．

填空题



Most taxpayers have had enough income tax collected by their employers during the year. So they do not (36) any money. In fact, most Americans get some money back. Last year, the Internal Revenue Service returned at least some money to more than (37) Americans paying individual income tax.
The United States has what is called (38) . Tax rates increase as (39) increase. This year, people earning more than (40) are taxed at the highest rate.
Earnings above that amount are taxed at (41) . Earnings below that amount are taxed at lower rates.
Individuals who earn less than (42) pay no income tax. But they do pay (43) , Medicare and other taxes.
There are many different ways for people and businesses to reduce their (44) Most homeowners, for example, can reduce their taxes a little by reporting to the I.R.S. the interest they pay (45) . This is called a (46)
In 2004, personal income taxes provided the government with most of its money: (47) of the budget. Social security and other (48) taxes provided (49) . Other forms of income include (50) , money borrowed to (51) and special taxes on (52) .
But income from taxes did not provide enough to pay for government spending. The I.R.S. says the budget deficit for 2004 was about (53) dollars.
Preparing tax documents can be complex. The I.R.S. estimates that taxpayers need an average of about 13 hours to prepare tax documents. And that is just for the basic tax form. For business, the I.R.S. estimates an (54) needs more than 50 hours. This is why a lot of Americans pay (55) to complete their tax documents for them.
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单项选择题

电压、电流、电阻之间关系（）

A.U=I／R

B.R=I/U

C.I=UR

D.U=IR

E.R=IU
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