请从以下两个小题中任选一个作答，若多选，则按所选的第一题计分．

A、正八边形的一个中心角的度数为____°．

B、用科学计算器比较大小： $\sqrt{13}$ cos20°____π．

“有权不可任性”既是现实的迫切呼唤，又是中央的明确要求。长期以来，受“权大于法”思想的影响和干部队伍素质的制约，A市行政审批过程中存在着“多、乱、滥”的现象，严重损害了政府的执行力和人民的利益。2015年以来，该市政府积极推进社会主义核心价值观教育，强化政策法规、业务知识等培训，使平等、公正、法治和服务的理念成为领导干部的自觉追求，实现了行政审批“高效率、零投诉”，从而使权力转变成造福民生的动力。

结合材料，运用“文化建设的中心环节”的有关知识，说明A市政府是怎样使权力变成造福民生的动力的？

当你的朋友问你们正在干什么的时候，你会说：

阅读下面短文。从短文后所给的A、B、C三个选项中选出最佳选项。

    Light travels at a very fast speed. It is about a million times 1 than the speed of sound. You can get some idea of this 2 by watching the start of a race (赛跑). If you stand some distance away from the starter, you can see smoke from his gun before the sound gets to your 3

    This great speed of light4some strange facts. Sunlight 5about 8 minutes to get to us. If you look at the light of the moon tonight, 6 that the light rays (光线) left the moon 1.3 seconds before they got to you. The nearest star is so 7 away that the light which you can see from it tonight started to 8 towards you several years ago. In some cases, the light from one of tonight's stars had started on its journey to you before you were born.

    So, to be exact, we can't say, "The stars are shinning 9." We have to say instead, "The stars 10 pretty. They were shining several years ago, but their light has only just got to the Earth."

已知 $4^a=7$ ， $6^b=8$ ，则 ${\text{log}}_{12}21$ 可以用 $a,b$ 表示为（    ）