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问题 选择题

已知R1：R2=2：1，则它们（　　）

A．串联时的电流之比为2：1

B．串联时的功率之比为1：1

C．并联时的电流之比为2：1

D．并联时的电压之比为1：1

答案

A、串联电路中，电流相等，电流之比1：1．不符合题意．

B、串联电路中，

P1
P2
=
I2R1
I2R2
=
R1
R2
=
2
1
．不符合题意．

C、并联电路中，

I1 
I2 
=
U
R1
U
R2
=
R2
R1
=
1
2
．不符合题意．

D、并联电路中，电压相等，电压之比为1：1．符合题意．

故选D．

完形填空
完形填空。

      There are different kinds of trains in the world. Trains can       1       people, animals, and other things.
Some trains are      2     , the others are slow. They can reach most of the cities and towns. Some trains go
    3    the hills if they are too high to go over. In big cities, the streets are usually     4     and some cars and
buses often have to wait.
      So you can find trains under the      5       and trains on a rail above the streets. People can quickly be
brought in or taken out. Can a train run over       6     ? Yes, it can.  There are many bridges over the
rivers.  But it is not easy to      7       bridges. Today's trains have dining-rooms for people. You
may      8     something in them on the way if you are hungry. Trains are      9      to men, women and
children. Many children like to play with model trains. With the model trains they can build their own rails
and      10       thewonderful world of trains.

( )1. A. carry    
( )2. A. help      
( )3. A. through  
( )4. A. full      
( )5. A. seas      
( )6. A. water    
( )7. A. mend      
( )8. A. cook      
( )9. A. useful    
( )10.A. hate      		B. take          
B. expensive      
B. among          
B. dirty          
B. farm          
B. air            
B. watch          
B. eat            
B. polite        
B. are afraid of  		C. bring            
C. fast            
C. during          
C. free            
C. ground          
C. forests          
C. reach            
C. find            
C. bad              
C. get on well with 		D. hold      
D. empty    
D. above    
D. busy      
D. gardens  
D. mountains
D. build    
D. pick      
D. lucky    
D. enjoy    
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选择题

命题“若一个数是负数，则它的平方是正数”的逆命题是[ ]

A．“若一个数是负数，则它的平方不是正数”

B．“若一个数的平方是正数，则它是负数”

C．“若一个数不是负数，则它的平方不是正数”

D．“若一个数的平方不是正数，则它不是负数”
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