如图，点A、B在⊙O上，且AB=BO．∠ABO的平分线与AO相交于点C，若AC=3，则⊙O的周长为 ____ 结果保留π）

已知：2NO₂+2NaOH→NaNO₃+NaNO₂+H₂O

NO+NO₂+2NaOH→2NaNO₂+H₂O

（以下涉及到的气体体积均为标准状况下数据）：

阅读下面短文，从A、B、C、D四个选项中选出最佳选项。

    David is a good student at school, but he is not clean. His face and hands are often dirty. He washes them with water only. He doesn't like to use soap(肥皂). He has an aunt. His aunt comes to see him. She looks at David and says, " David, I know what you ate for breakfast this morning." "What?" David looks at his aunt and asks.

"Eggs," says his aunt. "Your face and mouth tell me that."

    "You are wrong, my dear aunt!" says David. "Not today or yesterday, but the day before yesterday."

①实验后，某试管中溶液呈浅绿色，该试管中发生反应的化学方程式为____．

②实验后，向某试管中滴加无色酚酞，溶液变红，再向此试管中加稀盐酸，只观察到溶液由红色变为无色．据此推断，该试管中最初发生的反应的化学方程式可能为：____（写一个即可）．

③将②反应后的溶液，与上述五支试管中某试管反应后的溶液混合，看到先产生气体，后出现沉淀．则混合后溶液中溶质可能为____（写一种即可）．

④科学实验提倡绿色环保，对实验装置进行微型化改进是一条很好的途径．如图甲是实验时制取并检验CO₂的装置，图乙是对图甲实验装置微型化改进后的装置．

I．图乙中微型塑料滴管在实验中的作用与图甲中的____（填字母编号）仪器相同；

Ⅱ．通常用甲装置完成该实验需要的盐酸是微型实验装置用量的10倍，采用微型实验装置具有的优点是____．