답변을 얻은 뒤 반드시 감사의 덧글을 남기고, 좋은 답변은 채택해 주시는 센스...
질문할 때 예의를 지키시기 바랍니다.(빨리... 고수분만..... 이런 식의 요구는 자제 바랍니다.)
질문 내용 : 40. 글의 흐름으로 보아, 주어진 문장이 들어가기에 가장 적절한 곳은? Caffeine mimics this process; so it can “plug up” the sense organ and prevent adenosine from getting through.

If you start the day with a strong cup of coffee or tea, you’re not alone. Americans take in the caffeine equivalent of 530 million cups of coffee every day. Caffeine is the world’s most popular mood-altering drug. ( ① ) It’s also one of the oldest: according to archaeologists, man has been brewing beverages from caffeine-based plants since the Stone Age. (②) Caffeine doesn’t keep you awake by supplying extra energy; rather it fools your body into thinking it isn’t tired. ( ③ ) When your brain is tired and wants to slow down, it releases a chemical called adenosine. ( ④ ) Adenosine travels to special cells called receptors, where it goes to work counteracting the chemicals that stimulate your brain. ( ⑤ ) The result is that your brain never gets the signal to slow down, and keeps building up stimulants.

Adenosine travels to special cells called receptors, where it goes to work counteracting the chemicals that stimulate your brain.

여기서 the chemicals 로 왜 앞의 a chemical 에서 복수로 변하엿나요 전혀 그럴근거가없는것같은데.. 아니면 다른 chemical 인가요 그리고it 이 가리키는 게 뭔가요 앞에 가리킬 게 adenosine 이라는 단수밖에 없는 것 같은데 adenosine 을 가리키면 a chemical called adenosine에 위배되어 아데노신이 화학요소인데 자신이 자신을 counteract 할수없자나요 정확히 it과 the chemicals 가 무엇을 지칭하는지 알고싶습니다!