The chemical adenosine is released by brain cells when those cells are active.
Adenosine then binds to more and more sites on cells in certain areas of the
brain, as the total amount released gradually increases during wakefulness.
During sleep, the number of sites to which adenosine is bound decreases. Some
researchers have hypothesized that it is the cumulative binding of adenosine to a
large number of sites that causes the onset of sleep. Which of the following, if true, provides the most support for the researchers’
hypothesis? A Even after long periods of sleep when adenosine is at its lowest concen-
tration in the brain, the number of brain cells bound with adenosine remains
very large.
B Caffeine, which has the effect of making people remain wakeful, is known to
interfere with the binding of adenosine to sites on brain cells.
C Besides binding to sites in the brain, adenosine is known to be involved in
biochemical reactions throughout the body.
D Some areas of the brain that are relatively inactive nonetheless release
some adenosine.
E Stress resulting from a dangerous situation can preserve wakefulness even
when brain levels of bound adenosine are high.
答案是B, onset of sleep，那个onset 是理解为开始吗？如果是这样．那么光看那个hypothesis，我对B选项没有问题． 但是我怎么就看不懂在提出假设前的那一段话和那个假设有什么关系呢？估计是我理解能力太差了．麻烦哪位帮忙讲讲． | 
