①氧气②水③二氧化锰④碳⑤五氧化二磷．

在各项均为正数的等比数列$\left\{a_n\right\}$中,公比$q\in (0,1)$．若$a_3+a_5=5$,$a_2{a}_6=4$ ,$b_n={\log }_2{a}_n$数列$\left\{b_n\right\}$的前$n$项和为$S_n$,则当$\frac{S_1}{1}+\frac{S_2}{2}+...+\frac{S_n}{n}$取最大值时,$n$的值为（   ）

如图所示，小芳用画正方形的办法画出下列一组图案，你能按规律继续画下去吗？想想其中有哪些相似图形？

西汉宣帝说：“吾家自有制度，霸王道杂之。”这反映了西汉统治者自汉武帝以来统治思想特点是

根据句意及首字母提示完成单词

① My grandpa is ninety years old. He can’t see anything clearly, so he often wears g____.

②I always feel n____ when there is a big exam.

③Things are looking p____ good!

④My mother often tells us not to talk with s____. Because we don’t know them well.

⑤—May I speak to Alice, please?

—Sorry she’s out at the moment. Can I take a m____ for her?