”小安“，“悠悠”怎么破这样的阅读题？

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OG13 (Page 396）中的 一篇阅读（算是名篇吧），Q76

It was once believed that the brain was
independent of metabolic processes occurring
elsewhere in the body. In recent studies, however,
we have discovered that the production and release
(5) in brain neurons of the neurotransmitter serotonin
(neurotransmitters are compounds that neurons use
to transmit signals to other cells) depend directly on
the food that the body processes.
Our first studies sought to determine whether
(10) the increase in serotonin observed in rats given
a large injection of the amino acid tryptophan
might also occur after rats ate meals that change
tryptophan levels in the blood. Wefound that,
immediately after the rats began to eat, parallel
(15) elevations occurred in blood tryptophan, brain
tryptophan, and brain serotonin levels. These
findings suggested that the production and release
of serotonin in brain neurons were normally coupled
with biood-tryptophan increases. In later studies we
(20) found that injecting insulin into a rat's bloodstream
also caused parallel elevations in blood and brain
tryptophan levels and in serotonin levels. We then
decided to see whether the secretion of the animal's
own insulin similarly affected serotonin production.
(25) We gave the rats a carbohydrate-containing meal
that we knew would elicit insulin secretion. As we
had hypothesized, the blood tryptophan level and
the concentrations of tryptophan and of serotonin
in the brain increased after the meal.
(30) Surprisingly, however, when we added a large
amount of protein to the meal, brain tryptophan
and serotonin levels fell. Since protein contains
tryptophan, whyshould it depress brain tryptophan
levels? The answer lies in the mechanism that
(35) provides blood tryptophan to the brain cells. This
same mechanism also provides the brain cells with
other amino acids found in protein, such as tyrosine
and leucine. The consumption of protein increases
blood concentration of the other amino acids much
(40) more, proportionately, than it does that of tryptophan.
The more protein is in a meal, the lower is the ratio
of the resulting blood-tryptophanconcentration to
the concentration of competing amino acids, and
the more slowly is tryptophan provided to the brain.
(45) Thus the more protein in a meal, the less serotonin
subsequently produced and released.


Q76：
Accordingto the passage, the speed with which
tryptophan is provided to the braincells of a rat varies
with the
(A) amount of protein present in a meal
(B) concentration of serotonin in the brain before
a meal
(C) concentration of leucine in the blood rather than
with the concentration of tyrosine in the blood
after a meal
(D) concentration of tryptophan in the brain before
a meal
(E) number of serotonin-containing neurons

如果第一遍读原文采用略读，提取文章结构的方法。那么做到这题时除了A选项可能有印象能马上定位，E选项能直接排除。BCD估计得花很长的时间回原文定位，对比，判断。这样做题时间很难保证在1分半时间。

请教NN，大家做这道OG题时做题思路都是怎么样的呢？

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有人帮忙回答一下吗?