C is the best answer. Lines 11-23 state that adenosine “depresses neuron firing” by binding to specific receptors on neuronal membranes, which in turn inhibits the release of neurotransmitters. Lines 27-35 describe Snyder et al's hypothesis about caffeine. They propose that caffeine binds to specific receptors on neuronal membranes, which prevents adenosine from binding to those receptors and “allows the neurons to fire more readily that they otherwise would”. Therefore, according to Snyder et al, caffeine differs from adenosine in that caffeine permits neurotransmitter release when it is bound to adenosine receptors, whereas adenosine inhibits neruo transmitter release.