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    If you are looking for an animal to take the title of "most violent fish in the sea", then the tiger puffer

(河豚) fish would have to be a strong contestant. Not only is it deadly poisonous - though that doesn't

stop people trying to eat it - but it is also able to scare off enemies by inflating itself to become much

larger than normal, when it is young it even chews on its own brothers and sisters.

     Tiger puffer fish attach their eggs to rocks near the bottom of the sea, often at the mouths of bays.

Then the larvae (幼鱼) move to the entries of rivers and lakes once they have grown a little. Then,

having put on a lot of weight, they head out to sea. There's no innocent childhood for the puffer fish, as

Shin Oikawa of Kyushu University in Japan and his colleagues found out when they put the larvae of

tiger puffer fish in the lab and monitored them for two months. They found that the larvae went through

three steps in which their metabolic(新陈代谢) rates increased dramatically when they reached body

weights of 0.002g, 0.01g, and 0.1g. When a larva went through one of these steps, its behaviour also

changed. For instance, once a larva had passed the first level it would have grown its first tooth and

could start attacking larvae that had not yet reached that stage. Similarly, any larva that had reached

the 0.01g or 0.1g levels would start attacking lighter larvae.

     The researchers noted that the baby fish had a "relatively small mouth", so rather than swallowing

their brothers and sisters whole, they would bite pieces out of them. Despite this limitation, the fish

caused plenty of deaths -up to 12 per cent of the deaths that happened in the lab each day.

Those fish that grew fast enough to be able to chew on their fellows had an advantage. The extra food

accelerated their growth and development. Tiger puffer fish are likely to be faster and swifter, so they

can deal better with enemies.

     As the name suggests, puffer fish can inflate to make themselves seem much larger than they really

are, thus scaring off enemies. They do this by filling their stomachs, which are extremely elastic(弹性的),

with water. If that's not enough of a threat, the tiger puffer fish -like most of the other puffer fish in the

family -carries a deadly toxin(毒素). Eat one puffer fish and the poison will paralyse (瘫痪) your muscles,

including the muscles responsible for breathing, so death is usually caused by a lack of oxygen. Famously,

the fish is a delicacy in Japan, where highly qualified chefs produce dishes that contain the safe level of the poison. Interestingly, the puffer fish does not go to the trouble of producing the poison itself. Instead, it

hosts bacteria (细菌) that produce the stuff. It obtains these bacteria from its diet, so the youngest adult

fish are not poisonous.

1. All of the following statements account for the violence of puffer fish except that_____.

A. they are very difficult to catch

B. they can become larger to threaten enemies away

C. they kill their younger companions as they grow up

D. they are deadly poisonous

2. We can learn from the second paragraph that puffer fish _____.

A. can only grow to a weight of 0.1g

B. do not change their behavior as they grow up

C. like to find a safe place to hatch the next generation

D. begin to grow teeth when they reach the third level

3. Where does the poison in the puffer fish's body come from?

A. Its inner organs.          

B. The air it breathes in.

C. The diet it eats.          

D. The bacteria around it.

4. Which of the following statements is TRUE according to the passage?

A. Puffer fish can only produce poison when they face danger.

B. People like eating puffer fish even though they are poisonous.

C. The larvae are more aggressive than adult puffer fish.

D. Young puffer fish taste better than adult puffer fish.

在四端网络中，互成镜像关系的阻抗为四端网络的（）。

数字信号处理器由于在其内部设计了能够高速处理多路数字信号的电路，可以用在需要快速处理大量复杂数字信号的领域。下列哪一个设备不需要数字信号处理器

A．雷达

B．彩色电视机

C．数字音视频设备

D．数字图像处理设备

在二极管桥式整流电路中，若和负载并联一个滤波电容，则输出电压将（）。

A.增加

B.减小

C.不变

D.变为零

用热收缩片补口，要求钢管预热温度为（）。