Answer:
Adenosine
Step-by-step explanation:
According to the text, caffeine is a(n) _Adenosine_ inhibitor.
Adenosine is chemical secreted in the body that felicitates sleep and dilates the blood vessels, probably to ensure good oxygenation during sleep. Whereas, Caffeine acts as an adenosine-receptor antagonist. which means that it binds to these receptors, but it does not reduce the neural activity.