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问题 单项选择题

硼和镁形成的化合物刷新了金属化合物超导温度的最高记录。如右图是该化合物的晶体结构单元：镁原子间形成正六棱柱，且棱柱的上下面还各有一个镁原子；6个硼原子位于棱柱的侧棱上，则该化合物的化学式可表示为（）。

A.MgB

B.Mg3B2

C.MgB2

D.Mg2B3

答案

参考答案：B

解析：

填空题

大型拖拉机的履带做得很宽，这是为了__________；其表面做得凹凸不平是为了___________。
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填空题

Americans’ (1) impulses keep generating surprises. Charitable giving plays an even larger role in the (2) . Demand for nonprofit services gets proportionately bigger as a locality’s (3) rises. The philanthropy of the wealthy may not hinge on tax (4) to the degree many believe. The US (5) the world in levels of charitable activity. Some experts see charity as a (6) trait of the US, more than (7) or business. But those forces may be (8) , as many nonprofits, from healthcare to classical music are selling (9) in a marketplace alongside for-profit (10) . Charity is no mere (11) activity. It pays off for society in ways that may (12) the rates of return on many traditional investments. Charity not only helps those on the (13) end but also strengthens the (14) of society at large. Moreover, it appears to make the givers themselves more (15) . The pattern that conservatives are better givers than (16) is less about politics than about charity-linked (17) most common to conservatives, religious commitment, marriage and children, and entrepreneurship. The main point is that more Americans, regardless of ideology, embrace giving as a tool for (18) . The urge to make a difference, and to take (19) in it, outweighs (20) considerations.
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