题干

一个垂直于磁场方向射入匀强磁场的带电粒子(不计重力)的径迹如图所示,由于对周围气体的电离作用,带电粒子的能量越来越小,但电量未变,则可以断定(    )

A:粒子带负电,从b点射入

B:粒子带正电,从b点射入

C:粒子带负电,从a点射入

D:粒子带正电,从a点射入

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答案(点此获取答案解析)

A

同类题1

阅读下面短文,掌握其大意,然后从第1—20 各题所给的四个选项(A、B、C 和D)中,选出最佳选项。

    On a cold winter evening, I was walking home, tired and hungry, when suddenly came screams(呼喊声) from behind some tress.1, I slowed down to listen, and 2 that what I was hearing were the sounds of a fight. A woman was being3not far from where I stood.

    “Should I go to help her?” I thought. I was worried about my own 4,and I regretted having suddenly decided to take a new way home that night.“What if I am5too? Shouldn't I just6to the nearest phone and call the police?” But already the girl's7were growing weaker. I knew it was time for me to act fast because I could not8the life of this unknown woman,9 it meant risking my own life.

    I am not a 10 man. I didn't know where I found the11and physical strength(体力), but once I had finally12to help the girl, I became suddenly changed. I ran behind the bushes and 13the attacker off the woman. Struggling, we fell to the ground, where we fought for a few minutes 14the man jumped up and ran away.

    15 heavily,I stood up and came up to the girl, who was crying behind a tree. In the 16,I could not see her clearly, but I could certainly17her shock. Not wanting to frighten her any further, at first I spoke to her from a distance.

    “It's OK,” I said 18.“The man ran away. You're safe now.”There was a long stop and then I heard the words, said in both wonder and 19“Dad, is that you?” And then, from behind the tree, stepped my youngest 20, Katherine.

同类题2

While shopping, people sometimes can’t help ____into buying something they don’t really need.