什么是电位叠加原理？

已知全集U={3，6，k²+3k+5}，A={3，k+8}，则C_UA=______．

根据短文内容，用括号内所给单词的正确形式填空。

     Swimming is very popular in summer. People like swimming in summer because water (1) ________

(make) people feel cool. If you like swimming and swim in a wrong place, it may not be safe. Last summer,

more than ten people died while they (2) ________ (enjoy) themselves in the water, and (3) ________ (many)

of them were students. But some people are still not careful in swimming. They often think they swim so well

that nothing can happen to them in water. Summer is here again. If you go swimming in summer, don't forget

that better (4) ________ (swim) have died in water. They died only because they were not careful, not

because they could not swim. So don't get into water when you go swimming yourself, go with your teachers

or your parents. If there are"No swimming" signs, don't get into water, either. If you remember these,

swimming will be (5) _______ (safe).

书面表达

在家里学着自己做东西吃，那真是其乐无穷啊！特别是吃着自己炒的菜，自己做的包子、饺子时，尽管咸淡不均、味道欠佳，但也觉得特别香甜，请同学们回忆一下，最近曾在家里学着做过什么“美味佳肴”？谁教你的？操作步骤是怎样的？有何感受？

请以I can make…now!为题写一篇短文。词数在70左右。

关于钾的叙述，不正确的是（）

A.钾主要分布在细胞内液，只有2%在细胞外液

B.血浆钾浓度要比血清钾浓度高约0.5mmol/L左右

C.细胞内、外液中钾的浓度巨大差别主要依赖细胞膜上的钠泵作用

D.血清钾并不能准确反映体内总钾量

E.血清钾高于5.5mmol/L以上，称为高血钾

流体流动时，由于流层彼此间的相互作用而形成了流体的内摩擦力，从而使流体显示出粘滞性，这叫做流体的（）。

A、液位 

B、粘度 

C、流速 

D、内阻力