三七止血机制是（） A．增加血液凝血酶而止血  B．增加血小板数量而止血  C．使局

It may seem easy to say that plants and animals are different. Any child can tell the difference between  a tree and a dog. But those differences are far deeper than just what they look like. The cells (细胞) that plants are made up of are different. In other words, the cell wall of plants has something special.

First of all, it's important to find that not all plant cells have a cell wall. Besides, there are many differences of a cell wall based on the kinds of plant. The following is meant to be what a plant cell wall is  like.

The cell wall of a plant is on the outside of the cell membrane (膜). It is usually hard and cannot be bent easily, but the degree of flexibility (韧性) of the cell wall is different. It is based on the special use that the cell has in keeping the plant alive. Cells that make up the trunk of a tree are clearly less flexible than  cells on a leaf.

Cell walls act in many ways. They serve to protect the plant. They provide support, as well as help in the movement of chemicals and wastes into and out of the cell.  More importantly, they control the water  that goes in and out.  Plants must have good water controlling systems (系统), allowing them to keep water during dry seasons, and get rid of unnecessary water. Plants can't walk around to find water, so  being able to control water is very important.

So there you have it , what a plant cell wall is. Of course, there are many other things that are not talked about here. Knowing more about plant cell wails requires further study and research.

小题1:From the first two paragraphs we can find that _____.

A．all plants have a ceil wall

B．plant cells and animal cells are the same

C．different kinds of plant have different cell walls

D．there are no differences between the cells of a tree and a dog小题2:The underlined word "bent" in the passage is the closest in meaning to _____.

A．变形的

B．防水的

C．太阳灼伤的

D．表皮破损的小题3:Cells in the trunk of a tree are _____ those on a leaf.

A．as flexible as

B．not so flexible as

C．more important than

D．less important than小题4:What is the most important use of a plant cell wall?

A．To protect the plant.

B．To provide support.

C．To move chemicals and wastes.

D．To control the water.

（）是以社会折现率或财务基准收益率分别计算各方案的净流量等额年值，并进行比较，以净年大的方案为优。

A．净现值比较法

B．现值比较法

C．净年值比较法

D．年值比较法

把下面的六个图形分为两类，使每一类图形都有各自的共同特征或规律，分类正确的一项是：（）

A．A

B．B

C．C

D．D

经口气管插管时，要求（）、（）、（）三轴线重叠成一条线。

某商场举行促销活动，单间商品满300元减100元，满200元减50元，满100元减20元，若不参加活动则打8.5折，则买价值320元、240元、160元的商品各一件，最少需要（）元。

A.540

B.546

C.550

D.554