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?

• AEven after long periods of sleep when adenosine is at its lowest concentration in the brain, the number of brain cells bound with adenosine remains very large.  
• BCaffeine, which has the effect of making people remain wakeful, is known to interfere with the binding of adenosine to sites on brain cells.  
• CBesides binding to sites in the brain, adenosine is known to be involved in biochemical reactions throughout the body.  
• DSome areas of the brain that are relatively inactive nonetheless release some adenosine.  
• EStress resulting from a dangerous situation can preserve wakefulness even when brain levels of bound adenosine are high.