1. The primary purpose of the passage is to
（A） discuss a plan for investigation of a phenomenon that is not yet fully understood
（B） present two explanations of a phenomenon and reconcile the differences between them
（C） summarize two theories and suggest a third theory that overcomes the problems encountered in the first two
（D） describe an alternative hypothesis and provide evidence and arguments that support it
（E） challenge the validity of a theory by exposing the inconsistencies and contradictions in it 2. Which of the following， if true， would most weaken the theory proposed by Snyder et al？
（A） At very low concentrations in the human brain， both caffeine and theophylline tend to have depressive rather than stimulatory effects on human behavior.
（B） The ability of caffeine derivatives at very low concentrations to dislodge adenosine from its receptors in mouse brains correlates well with their ability to stimulate mouse locomotion at these low concentrations.
（C） The concentration of cyclic AMP in target neurons in the human brain that leads to increased neuron firing can be produced by several different phosphodiesterase inhibitors in addition to caffeine.
（D） The concentration of caffeine required to dislodge adenosine from its receptors in the human brain is much greater than the concentration that produces behavioral stimulation in humans.
（E） The concentration of IBMX required to dislodge adenosine from its receptors in mouse brains is much smaller than the concentration that stimulates ocomotion in the mouse.
3. According so Snyder et al， caffeine differs from adenosine in that caffeine
（A） stimulates behavior in the mouse and in humans， whereas adenosine stimulates behavior in humans only
（B） has mixed effects in the brain， whereas adenosine has only a stimulatory effect
（C） increases cyclic AMP concentrations in target neurons， whereas adenosine decreases such concentrations
（D） permits release of neurotransmitters when it is bound to adenosine receptors， whereas adenosine inhibits such release
（E） inhibits both neuron firing and the production of phosphodiesterase when there is a sufficient concentration in the brain， whereas adenosine inhibits only neuron firing