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问题 选择题

物体M从A运动到B，前半程平均速度为v1，后半程平均速度为v2，那么全程的平均速度是（　　）

A．

v1+v2
2

B．

v1•v2

C．

v12+v22
v1+v2

D．

2v1v2
v1+v2

答案

设全程位移为s，则前半程运动的时间t1=

s
2
v1
，后半程运动的时间t2=
s
2
v2
，则全程的平均速度
.
v
=
s
t
=
s
t1+t2
=
2v1v2
v1+v2
．故D正确，A、B、C错误．

故选D．

完形填空
完形填空

       The bicycle is one of the simplest (最简单的) yet most useful inventions in the world. What is most
surprising is that it was not invented earlier,   1   the great inventor Leonardo da Vinci had drawn pictures
for bicycles and also for flying machines and some other things. Those things  were not produced   2   long after he died.
       A person riding a bicycle use   3   energy(能量)to make the bicycle move, and there is no pollution
at all when you are riding. Even so, m developed   4  , most people don't travel to  work by bicycle. It is
not because the bicycles are expensive or people feel   5   if they ride to work. It's because    6   cars on
the roads becomes larger. It certainly becomes   7   to ride a bi-cycle. As a result, more people put their
bicycles away and go to work   8   their cars and, in this way, the situation(情形)is made more serious.  9   the best way to make riding safer and more popular is to create paths(开设通道)only for bicycles, and
to make   10    so difficult and expensive for drivers to take their cars into the city that they will go back
to use their bicycles.


( )1. A．although          
( )2. A．when              
( )3. A．much              
( )4. A．world            
( )5. A．comfortable      
( )6. A．the number of    
( )7. A．safer            
( )8. A．by                
( )9. A. Always            
( )10.A．it                
                                          		B．so              
B．before          
B．quite a lot of  
B．countries        
B．1ucky            
B．a number of      
B．more dangerous  
B．in              
B．May be          
B．them          
C．but              
C．since            
C．very little      
C．1and            
C．sorry            
C．this kind of    
C．much dangerous  
C．use              
C．Perhaps          
C．us               
                                		D．because         
D．until           
D．many            
D．earth           
D．tired           
D．a kind of       
D．safe            
D．drive           
D．Nearly          
D．that      
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选择题

滑动变阻器能够改变电阻，是靠改变导线的[ ]

A．粗细

B．接线柱

C．长度

D．温度

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