赫格隆皮带液压驱动系统启动皮带时，先启动冲洗电机，再启动主电机，最后启动皮带，调节皮

知识库子系统包括（）

A.信息库

B.数据库

C.知识库

D.知识库分类系统

军团菌肺炎与下列何种因素关系密切（）

A.中央空调污染

B.食品污染

C.饮用水污染

D.土壤污染

E.患者污染

阅读理解。

     Animals can move from place to place, but plants cannot. When an animal is under attack, it can run

away or fight back. Plants certainly cannot run away, and they lack teeth and claws. But plants can

defend (保护) themselves by using both physical and chemical means.

     Some plants have their own ways to keep animals away. For example, the leaves of the holly plant

have sharp spines (刺) that discourage grass-eating animals. Holly leaves on lower branches have more

spines than leaves on upper branches. This is because the lower leaves are easier for most animals to

reach.

Some plants, such as the oak tree, have thick and hard leaves that are difficult for animals to eat. Some

grasses may contain a sandy material; eating such grasses wears down the animal's teeth.

     Many plants also have chemical defenses. Some plants produce chemicals that taste bitter or cause an

unpleasant reaction. Some plants may fight against an attack by increasing the production of these chemicals. When a caterpillar (毛虫) bites a tobacco leaf, the leaf produces a chemical messenger. This messenger sends to the roots the information to produce more nicotine. The higher levels of nicotine discourage the

caterpillar.

    Many plants depend on both physical and chemical defenses. A certain plant in China, for instance,

has prickly (多刺的) leaves, and each prickle contains poisonous venom (毒液) A single experience with

this kind of plant will teach an animal to stay away from it in the future.  

1. To defend themselves, oak trees use________.

A. chemical means

B. physical means

C. bitter chemicals

D. sandy materials

2. How does tobacco protect itself against an attack from a caterpillar?

A. Its leaves fight against the attack by physical means.

B. Its roots send a messenger to discourage the caterpillar.

C. Its roots increase the production of nicotine when it is attacked.

D. Its leaves produce poisonous sand to drive the caterpillar away.

3. What would be the best title for this passage?

A. Plants and Animals    

B. How Plants Defend Themselves

C. Attacks and Defenses    

D. How Animals Eat Plant Leaves

氧气瓶内的氧气不能全部用尽，充气前应留有( )kPa的压力。

A．50～100
B．100～150
C．150～200
D．200～265

刚度是指杆件在外力作用下不会发生任何变形。