维持生态平衡必不可少的生物是（　　）

血糖是指 ____ ，血糖的含量对人体的生命活动十分重要。在早晨进食前，测定人的血糖的含量约为 ____  。正常人的血糖浓度并不受饮食的原因出现很大的波动，因为当血液中葡萄糖含量上升时，____ 分泌增加，促使血糖含量  ____ ；当血液中葡萄糖含量下降时，____ 分泌减少，使血糖含量____ ，从而使血糖含量维持在正常水平。

阅读理解，判断对错

      Simon is a good boy. Today is Sunday and it is fine, so Simon's father takes him to visit the Great Wall. The Great Wall is far away from his home. So they are going there by bus. The Great Wall is about six thousand seven hundred kilometres. It's very long and great. Simon is very happy and his father is happy, too.

水循环包括自然循环和社会循环。读图，回答。

图中①②③④分别为（ ）