[求助]GWD-5-26逆否命题的判断,急切盼望各位的帮忙

carmenleaf

Q26:

Healthy lungs produce a natural antibiotic that protects them from infection by routinely killing harmful bacteria on airway surfaces.  eople with cystic fibrosis, however, are unable to fight off such bacteria, even though their lungs produce normal amounts of the antibiotic.  The fluid on a0.irway surfaces in the lungs of people with cystic fibrosis has an abnormally high salt concentration; accordingly, scientists hypothesize that the high salt concentration is what makes the antibiotic ineffective.

Which of the following, if true, most strongly supports the scientists’ hypothesis?

1. When the salt concentration of the fluid on the airway surfaces of healthy people is raised artificially, the salt concentration soon returns to normal.


2. A sample of the antibiotic was capable of killing bacteria in an environment with an unusually low concentration of salt.


3. When lung tissue from people with cystic fibrosis is maintained in a solution with a normal salt concentration, the tissue can resist bacteria.


4. Many lung infections can be treated by applying synthetic antibiotics to the airway surfaces.  


5. High salt concentrations have an antibiotic effect in many circumstances.

answer : c

有一个问题困扰我很久，借此题问一下各位：

结论：scientists hypothesize that the high salt concentration is what makes the antibiotic ineffective.

即：high salt concentration ——〉antibiotic ineffective.

那么与之等价的逆否命题则是：antibiotic effective.——〉normal salt concentration

而c选项：a normal salt concentration——〉tissue can resist bacteria则是原命题的否命题，与原命题不等价的呀，所以我坐看右看觉得都不对，不晓得是哪个地方分析错了。

   记得在上XDF的时候把费费给问郁闷了，没好意思再问，结果到现在还是不懂，麻烦大家指点一下呀，多谢多谢！

[此贴子已经被作者于2005-9-7 0:09:23编辑过]

carmenleaf

没有回帖...

还是没想通啊：A-->B 等价于  B(取反)-->A(取反)  不等价于 A(取反)-->  B(取反)

这个关系是对的吧？

carmenleaf

没有人来看吗？由于这个问题没搞懂，相关的题几乎都错了，急呀，拜托各位帮帮忙...ethyl,看到你的回帖就在我这一帖的上面止住，伤心...

[此贴子已经被作者于2005-9-7 0:07:10编辑过]