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问题 填空题
观察下列各式：
2
2-4
+
6
6-4
=2，
5
5-4
+
3
3-4
=2，
7
7-4
+
1
1-4
=2，
10
10-4
+
-2
-2-4
=2，依照以上各式形成的规律，在括号内填入正确的数______，使等式
20
20-4
+
()
()-4
=2成立．
答案

两个分子之和为8，一个分子为20，∴另一个分子=8-20=-12．

多选题
如图所示，实线为一匀强电场的电场线，一个带电粒子射入电场后，留下一条从a到b虚线所示径迹，重力不计，下列判断正确的是（　　）

A．b点的电势高于a点的电势

B．粒子在a点的电势能小于在b点的电势能

C．粒子在a点动能大于在b点的动能

D．场强方向向左
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单项选择题

Hardin argued that grazing land held in common (that is, open to any user) would always be used less carefully than private grazing land. Each rancher would be tempted to overuse common land because the benefits would accrue to the individual, while the costs of reduced land quality that results from overuse would be spread among all users. But a study comparing 217 million acres of common grazing land with 433 million acres of private grazing land showed that the common land was in better condition.
Which of the following, if true and known by the ranchers, would best help explain the results of the study ？（）

A．With private grazing land, both the costs and the benefits of overuse fall to the individual user.

B．The cost in reduced land quality that is attributable to any individual user is less easily measured with common land than it is with private land.

C．An individual who overuses common grazing land might be able to achieve higher returns than other users can, with the result that he or she would obtain a competitive advantage.

D．If one user of common land overuses it even slightly, the other users are likely to do so even more, with the consequence that the costs to each user outweigh the benefits.

E．There are more acres of grazing land held privately than there are held in common.
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