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问题 不定项选择

如图所示，Q和-Q是两个等量异种点电荷，以点电荷+Q为圆心做圆，A、B为圆上两点，MN是两电荷连线的中垂线；与两电荷连线交点为O，下列说法正确的是    

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A．A点的电场强度大于B点的电场强度    

B．电子在A点的电势能小于在B点的电势能       

C．把质子从A点移动到B点，电场力对质子做功为零   

D．把质子从A点移动到MN上任何一点，质子的电势能变化都相同

答案

BD

单项选择题

To round is to delete or (66) one or more of the least significant digits in a positional (67) and to, or not to, adjust the part retained in accordance with some specified rule. The purpose of rounding is usually to limit the (68) of the numeral or to reduce the number of characters in the numeral, or to do both these things.

The most common arithmetic forms of rounding are rounding down, rounding up and rounding off. Rounding down is defined as making no adjustment to the part of the numeral that is retained. Rounding up means adjusting the part of the numeral that is retained by adding 1 to the least significant of its digits and executing any necessary carries, if and only if One or more non-zero digits have been deleted. Rounding off (69) adjusting the part of the numeral retained by adding 1 to the least significant of its digits and executing any necessary carries, if and only if the most significant of the digits deleted was equal to or greater than half the (70) of its digit place.

（66）处填（）。

A．simplify

B．reduce

C．abbreviate

D．omit
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选择题

“神舟五号”宇宙飞船的成功发射，表明我国载人航天技术有了重大突破．运送“神舟五号”飞船的火箭所使用的燃料是偏二甲肼（化学式C2H8N2）下列说法正确的是（　　）

A．偏二甲肼是由碳、氢、氮三种元素组成的

B．偏二甲肼是由两个碳元素、八个氢元素、两个氮元素组成的

C．偏二甲肼中含有氮气

D．每个偏二甲肼分子中含有两个碳原子、八个氢原子、两个氦原子
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