一道关于充分必要假设的题目

Joseph_lin

22.   In humans, ingested protein is broken down into amino acids, all of which must compete to enter the brain. Subsequent ingestion of sugars leads to the production of insulin, a hormone that breaks down the sugars and also rids the bloodstream of residual amino acids, except for tryptophan, Tryptophan then slips into the brain uncontested and is transformed into the chemical serotonin, increasing ht brain’s serotonin level. Thus sugars can play a major role in mood elevation, helping one to feel relaxed and anxiety-free.

Which one of the following is an assumption on which the argument depends?
(A) Elevation of mood and freedom from anxiety require increasing the level of serotonin the brain.
(B) Failure to consume foods rich in sugars results in anxiety and a lowering of mood.
(C) Serotonin can be produced naturally only if tryptophan is presented in the bloodstream.
(D) Increasing the level of serotonin in the brain promotes relaxation and freedom from anxiety.
(E) The consumption of protein-rich foods results in anxiety and a lowering of mood.

答案是D，我看了下Critical Bible上的解释就是说A不行的原因就是把increase in serotonin当成了必要条件，而原文是充分条件。但是我觉得如果我对A取非，就是Elevation of mood and freedom from anxiety doesn't require increasing the level of serotonin the brain,那么不也削弱了原文的结论Increasing the level of serotonin in the brain promotes relaxation and freedom from anxiety了吗，这样应该也可以吧？？？
如果我这里再多出2个选项：F No Elevation of mood and freedom from anxiety, No increasing the level of serotonin the brain 和 G No increasing the level of serotonin the brain, No Elevation of mood and freedom from anxiety,那么按照上面的充分和必要的说法的话，是不是也只有F对了呢？

Joseph_lin

UP

kikisunflower

A is out of scope since it discusses whether increasing the level of serotonin the brain is necessary to elevate mood situation whereas the argument focuses on a static condition.

It is likely that elevation of mood can happen when the level of serotonin remains unchanged.

ageng

require取非不是not require。而是不一定require。正如must取非应该是might not，而不是not。
仅个人观点。感觉取非虽然是assumption的利器，但很容易绕进去。有时候都不知道该怎么取非了。

sdcar2010

Necessary assumption. Use negation.

If you negate A), you have:
Elevation of mood and freedom from anxiety does not require increasing the level of serotonin the brain.
If this is true, then elevation of mood and freedom requires something other than increasing level of S in the brain. However, the fact that increasing level of S is not a necessary condition for relaxation does not mean it is not a sufficient condition therefor.
Thus, the conclusion still holds.

In terms of formal logic, the assumption here is S in brain --> Rel; Choice A) is Rel -->S in brain. So A) is not the assumption. C) is since in terms of formal logic, it is Rel -->A in brain.

BTW
Premises:
1) Sugar --> Insulin
2) Insulin --> T in blood
3) T in blood --> T in brain
4) T in brain --> S in brain

Conclusion:
Sugar --> Rel

Assumption:
S in brain --> Rel

Joseph_lin

如果我这里再多出2个选项：F No Elevation of mood and freedom from anxiety, No increasing the level of serotonin the brain 和 G No increasing the level of serotonin the brain, No Elevation of mood and freedom from anxiety,那么按照上面的充分和必要的说法的话，是不是也只有F对了呢？