og-34问个笨问题

34. In 1985 state border colleges in Texas lost the enrollment of more than half, on average, of the Mexican nationals they had previously served each year. Teaching faculties have alleged that this extreme drop resulted from a rise in tuition for international and out-of-state students from $ 40 to $ 120 per credit hour.

Which of the following, if feasible, offers the best prospects for alleviating the problem of the drop in enrollment of Mexican nationals as the teaching faculties assessed it?

A. Providing grants-in-aid to Mexican nationals to study in Mexican universities.

B. Allowing Mexican nationals to study in Texas border colleges and to pay in-state tuition rates, which are the same as the previous international rate

C. Reemphasizing the goals and mission of the Texas state border colleges as serving both in-state students and Mexican nationals

D. Increasing the financial resources of Texas colleges by raising the tuition for in-state students attending state institutions

E. Offering career counseling for those Mexican nationals who graduate from state border colleges and intend to return to Mexico

key is B.虽然看上去很简单，但是用逻辑分析好像不对。

事件A:rise in tuition

事件B:enrollment drop

A->B

答案并不是该命题的逆否命题，而是否命题~A->～B。根据逻辑原理：原命题的否命题不一定正确，那怎么解释~A会推出~B呢？

tks!

请问：“Teaching faculties have alleged that this extreme drop resulted from a rise in tuition for international and out-of-state students”不算一个命题么？tuition rise不是enrollment drop的充分条件么？迷惑

还有，这道题好像和OG90一样,都属于携隐的总结中http://forum.chasedream.com/dispbbs.asp?BoardID=24&ID=70905中的6.      取非题：原文描述缺点问解决方法

OG90:

A: sustained massive use

B:kill natural enemy/give rise to insecticide-restriant pests

A－>B,答案是~A－>~B

仍旧是否命题就是答案了。

谢谢lawyer，好像有点明白了，只有if,if only,unless,require,depend on,must 等关键词出现，才算作是充分必要条件，没有这些关键词的话，只能算因果关系，不能作为充分必要条件来进行逻辑推理，是这样吧？