1 . How does an ecosystem (生态系统) work? What makes the populations of different species the way they are? Why are there so many flies and so few wolves? To find an answer, scientists have built mathematical models of food webs, noting who eats whom and how much each one eats.

With such models, scientists have found out some key principles operating in food webs. Most food webs, for instance, consist of many weak links rather than a few strong ones. When a predator (掠食动物) always eats huge numbers of a single prey (猎物), the two species are strongly linked; when a predator lives on various species, they are weakly linked. Food webs may be dominated by many weak links because that arrangement is more stable over the long term. If a predator can eat several species, it can survive the extinction (灭绝) of one of them. And if a predator can move on to another species that is easier to find when a prey species becomes rare, the switch allows the original prey to recover. The weak links may thus keep species from driving one another to extinction.

Mathematical models have also revealed that food webs may be unstable, where small changes of top predators can lead to big effects throughout entire ecosystems. In the 1960s, scientists proposed that predators at the top of a food web had a surprising amount of control over the size of populations of other species—including species they did not directly attack.

And unplanned human activities have proved the idea of top-down control by top predators to be true. In the ocean, we fished for top predators such as cod on an industrial scale, while on land, we killed off large predators such as wolves. These actions have greatly affected the ecological balance.

Scientists have built an early-warning system based on mathematical models. Ideally, the system would tell us when to adapt human activities that are pushing an ecosystem toward a breakdown or would even allow us to pull an ecosystem back from the borderline. Prevention is key, which scientists says because once ecosystems pass their tipping point (临界点), it is remarkably difficult for them to return.

1. What have scientists discovered with the help of mathematical models of food webs?

A．The living habits of species in food webs.

B．The rules governing food webs of the ecosystems.

C．The approaches to studying the species in the ecosystems.

D．The differences between weak and strong links in food webs.

2. A strong link is found between two species when a predator ______.

A．has a wide food choice	B．can easily find new prey
C．sticks to one prey species	D．can quickly move to another place

3. What will happen if the populations of top predators in a food web greatly decline?

A．The prey species they directly attack will die out.

B．The species they indirectly attack will turn into top predators.

C．The living environment of other species will remain unchanged.

D．The populations of other species will experience unexpected changes.

4. How does an early-warning system help us maintain the ecological balance?

A．By getting illegal practices under control.

B．By stopping us from killing large predators.

C．By bringing the broken-down ecosystems back to normal.

D．By signaling the urgent need for taking preventive action.

2 . 阅读下面材料，根据其内容和所给段落开头语续写两段，使之构成一篇完整的短文。续写的词数应为150左右。

It was a bitterly cold and dark day in December. I was walking home from work, hungry and worn out after my long day. My team was due to complete our big project before we left for our winter vacation, but our boss delivered bad news that our client had contacted him and demanded a major revision on the project before they would accept it. The whole team was desperate—we were so close to finishing and now we would most likely have to work over and miss the entire vacation.

So caught up in my anxious thoughts was I that I failed to watch where I was walking until I stepped, with a dramatic splash (溅起), off the sidewalk and into a giant, half-frozen pool of water at the side of the road. I could feel the icy water into my shoes and socks, slowly soaking up into my trousers legs around my ankles. I sighed deeply and almost cried—I was having, as my grandmother always used to say, “one of those days when nothing goes your way.”

A wave of loneliness filled me. I loved my job and my coworkers and most days when I was satisfied with my life but now, especially around the holidays, the reminders that I didn’t have a partner or children like my sister or some of my coworkers did fill me with sadness. As I looked down sadly at my unpleasant situation, I saw a brief spot of movement out of the corner of my eye. There, attached to a cold piece of cardboard in the middle of the pool, was a small patch of grey fur.

As I watched, the little spot moved again, before letting out an outsized cry of sorrow and grief, so loud was it that you would think it came from a creature five times the size. It was a cat!

Paragraph 1:

Slowly and carefully so as not to frighten it, I bent down and picked up the cat.

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Paragraph 2:

My sister is a nurse, and therefore the family expert on caring for animals.

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3 . Namu is a Thai hairdresser. She is providing_______haircuts for medical workers who are fighting COVID-19 at hospitals in Bangkok. Namu began her free service in late March. In the_______, she was unsure about offering the_______, thinking it was unimportant. But now, she is_______with the warm welcome she receives from health care workers.

On her first hospital visit, Namu had_______cutting the hair of six health workers. But in fact she_______   haircuts to 20 of them at last. On her second visit, the number quickly, rose to 30. Namu said, “They just keep     _______. Some arrived and asked if they could have a_______right away as they had to go to work.” At that point, she asked her friends to_______her. Now, the team is usually provides about 50 haircuts each day.

Namu is just proud that her skill is making a________.“The medical workers are happy to get a haircut. I think they feel________and more relaxed. They all want to do something with their hair and________themselves for work,” she said.

For Dr. Bill, it is the humanity(人性) behind the haircut that is really________. He said, “I don’t feel like this is   ________a service I am getting. It feels like more of a________given by a group of people to our hospital staff. This is more than just a haircut.

1.

A．special

B．short

C．regular

D．free

2.

A．beginning

B．moment

C．mind

D．past

3.

A．chance

B．service

C．comfort

D．lesson

4.

A．busy

B．careful

C．content

D．familiar

5.

A．worked on

B．planned on

C．kept up

D．ended up

6.

A．showed

B．taught

C．brought

D．gave

7.

A．calling

B．moving

C．coming

D．visiting

8.

A．haircut

B．rest

C．look

D．try

9.

A．direct

B．help

C．trust

D．praise

10.

A．rule

B．deal

C．profit

D．difference

11.

A．funnier

B．lighter

C．safer

D．stronger

12.

A．calm

B．encourage

C．prepare

D．exchange

13.

A．important

B．reliable

C．useful

D．difficult

14.

A．mostly

B．hardly

C．generally

D．merely

15.

A．donation

B．promotion

C．kindness

D．promise