如图所示，D是一只二极管，它的作用是只允许电流从a流向b，不允_爱下问搜题

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问题选择题

如图所示，D是一只二极管，它的作用是只允许电流从a流向b，不允许电流从b流向a。平行板电容器AB内部原有带电微粒P处于静止状态，当两极板A和B的间距稍增大一些后，微粒P的运动情况将是：

A．向上运动 B．向下运动 C 仍静止不动 D．无法判断

答案

答案：C

题目分析：电容器与电源保持相连，电容器的电压不变，板间距离增大，电容减小，电容量的电量要减小，放电，但由于二极管的作用只允许电流从a流向b，电容器的电量无法减小，根据推论板间电场强度，Q、S、ɛ不变，则改变d，E不变，电荷P受的电场力不变，仍静止．故A正确．故选A

阅读理解

阅读下面短文，掌握其大意，然后从A、B、C、D四个选项中选出可以填入空白处的最佳选项。
At least thirty times Bobby had tried to climb the snowy mountain, but he had never   36   the top. His friend old Peeper, who knew a lot about his failures, 37 Bobby to try again. He gave Bobby a pair of sunglasses and said, “If it starts clouding over, put on the glasses, or if your feet start hurting, put them on too. The glasses are very 38 . They’ll help you.” Bobby received the gift without much thought.
The day came when he was to have another   39 . About two hours after he started, he felt his feet hurting. Then he 40 what Peeper had said, and put on the glasses. The pain was pretty bad, but   41  the sunglasses he could clearly see the snow-covered mountain top, so he went on.
  42 , clouds were gathering. But this time Bobby could still see the 43 through the clouds. He kept climbing , leaving the clouds behind, forgetting his pain, and finally arriving at the top. It was certainly worth it. His feeling of   44  was extremely good, almost as great as that 45 view. The mountain below was surrounded by a sea of clouds. He couldn’t believe that the clouds were as thick as that, so he looked more closely at the sunglasses, and discovered the 46 . Peeper had engraved (镌刻) the snow-covered mountain top on the sunglasses, 47 Bobby could see it when he was looking upwards!
Bobby __48  that the only difficulty to reaching the top had been losing   49  when he couldn’t see the mountain top. He was thankful that Peeper had used that little trick to help him see that his   50   was never impossible, and that it was still there, where it had always been.

C．Unfortunately

单项选择题

引起继发性再生障碍性贫血的常见抗生素药物是（）。

A.红霉素

B.链霉素

C.氯霉素

D.青霉素

E.土霉素