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问题 选择题

如图，在正在发生无阻尼振荡的LC回路中，开关接通后，电容器上电压的振幅为U，周期为T，若在电流为零的瞬间突然将开关S断开，回路中仍做无阻尼振荡，电容器上电压的振幅为U′，周期为T′，则（　　）

A．T=T′，U=U′

B．T＜T′，U=U′

C．T=T′，U＜U′

D．T＜T′，U＜U′

答案

LC振荡电路的周期为T=2π

LC
，将开关S断开，线圈L长度变大，故自感系数L变大，故周期变大；

电流为零时是充电完毕瞬间，磁场能量全部转化为电场能量，由于是无阻尼振荡，故最大电场能相同，故电压的振幅也相同；

故选B．

单项选择题

Are you a man or a mouse When people ask this question they want to know (1) you think you are a (2) person or a coward. But you will never really know the answer to this question (3) you are tested in real life. Some people (4) they are brave but when they come face to face with real (5) , they act like cowards. Others think of themselves as cowards, but when they meet danger, they act like (6) .Lenny had always thought of himself as a (7) person. He got worried before examinations. He worried about his job and health. All he wanted in life was to be safe and healthy. (8) , on January 15th, 2002, a plane crashed into the Potomac River in Washington. Lenny went to the river to See what was happening. He saw a woman in the (9) water. Lenny did not feel afraid. He kept very (10) and did a very dangerous thing. He jumped into the Potomac, (11) to the woman, and kept her head (12) the water. Seventy-eight people died that day. Thanks to Lenny, it was not (13) .When you are in a very (14) situation and feel afraid, the body automatically produces a chemical in the blood. The chemical is called adrenalin. (15) adrenalin in the blood system, you actually feel per and per and are (16) to fight or run away. However, when you are absolutely terrified, the body can produce too much adrenalin. Where this (17) , the muscles become very hard and you find you (18) move at all. You are then paralyzed with fear. This is (19) when we are very frightened, we sometimes say we’re "petrified". This word comes from the Greek word "petros", which means "stone". We are (20) frightened that we become stonelike.

 

第（14）空应选择（）

A．dangerous

B．comfortable

C．different

D．favorable
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单项选择题 A1/A2型题

细菌血清学试验中，通常有诊断意义的恢复期抗体效价比初次效价是（）。

A.≥2倍

B.≥3倍

C.≥4倍

D.≥5倍

E.≥6倍
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