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问题 填空题

一小球从离平面镜3 m处以0.2 m/s的速度沿垂直平面镜的方向向平面镜滚去，5 S后小球与镜中的像相距______m；在小球的运动过程中，像的大小如何变化？______．

答案

像到平面镜的距离与物体到平面镜的距离相等，5s后小球与平面镜的距离变为2m，像与平面镜的距离变为2m，则小球与镜中的像相距4m．平面镜所成的像与始终物体大小相等．

答案为：4，大小不变．

单项选择题

JZ-7分配阀主阀小膜板上方为：（）。

A、大气压力

B、制动缸压力

C、作用风缸压力
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单项选择题

Should we care if over 150 known species of animals have (91) from the earth in the last fifty years Should we be concerned that there are (92) thousands of species whose very existence is (93) endangered—largely because of our activities (94) , after all, is the natural end of populations. Species are born, then (95) , and then die. Some live a (96) time, perhaps millions of years; some die more quickly. We have (97) the extinction of many species we know about, and we have undoubtedly sealed the (98) of others. In fact, there are undoubtedly many other species that have lived among us during our time on earth, but that have disappeared as a (n) (99) of our activities without our ever having known they existed.
It is hard to explain the rationale of many of us who are concerned about such matters. I have never seen a sea whale, (100) I don’t want them to become extinct. Moreover, I felt this way long before I understood anything about how they might be an important part of an ecosystem. Possibly such feelings merely reflect the cultural attitude that it is "nice" to wish other living things well; thus, the attitude is (101) . I feel nice.
There are, of course, more rational reasons for (102) the extermination of any species. For one thing, the kind of attitude that encourages or sanctions the destruction of other species is a threat (103) our own wellbeing. If such an attitude exists, we ourselves might (104) victim to it. living things (including us) might be expected to fare better where there is (105) for life. The extinction of other species could also threaten us (106) by simplifying the system of which we are a part or by destroying parts of the ecosystem (107) which we directly rely. For example, if we continue to poison the oceans (108) we are willing to believe only a few bottom dwellers are affected, we might (109) overstep some critical threshold and trigger the wholesale death of plankton, thus finding ourselves without a major (110) of the world’s food and with our oxygen supplies dwindling.

A．Extinction

B．Disappearance

C．Cease

D．Death
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