题目内容：

    Fever, coughing, sore throat… most of us at one time or another have had these symptoms. What causes them?  In fact, many diseases are caused by viruses, including some deadly ones like AIDS, SARS and COVID-19, also known as the novel coronavirus pneumonia, outbreak in China.

What are viruses?  How can viruses cause so much trouble?  Viruses are very small particles (颗粒) which are about one millionth of an inch long. Unlike human cells or bacteria, viruses aren’t living organisms- they can't reproduce on their own. Instead, they invade the cells of living organisms to reproduce, spread and take over.

Virus can infect every living thing- from plants and animal down to the smallest bacteria. For this reason, they are always a potential danger to humans. Viruses lie around our environment all of the time, waiting for a host cell to come along. They can enter our bodies by the nose, mouth, eyes or breaks in the skin. Once inside, they try to find a host cell to infect. For example, HIV, which causes AIDS, attacks the T-cells of the immune system.

But the basic question is, where did viruses first come from? Until now, no clear explanation for their origin exists. "Tracing the origins of viruses is difficult, "Ed Rybicki, a scientist at the university of Cape Town in South Africa, told Scientific American, "because viruses don' t leave fossils and because of the tricks they use to make copies of themselves within the cells they've invaded.”

However, there are three main theories to explain the origin of viruses. First, viruses started as dependent organisms, then became parasites(寄生者)。Second, viruses evolved from pieces of DNA or RNA that "escaped" .from larger organisms. Third, viruses co-evolved with their host cells, which means they existed alongside these cells.

For the time being, these are only theories. The technology and evidence we have today cannot be used to test these theories and identify the most reasonable explanation. Continuing studies may provide us with clearer answers or future studies may find that the answer is even more mysterious than it now appears.

1.What can we learn about viruses from the text?

A.Viruses are really small living organisms.

B.Viruses have nothing to do with the common cold.

C.Viruses can't reproduce unless they find a host cell.

D.Viruses enter our bodies mainly through our mouths, noses and hair.

2.Why are viruses always a potential danger to humans?

A.TH most infect all the living things around us.

B.They prefer evolving with the cells in our body.

C.They can copy human's DNA and evolve successfully.

D.They can enter our body far more easily than other living things.

3.Which of the following might explain the origin of viruses?

A.They evolved from the DNA of animals.

B.They evolved along with their host cells.

C.They evolved from the fossils of large organisms.

D.They evolved from parasites to independent organisms.

4.What can we conclude from the last paragraph?

A.The author is confident about future virus research.

B.Viruses will become more like bacteria as they evolve.

C.Viruses live longer in human host cells than in animals.

D.It may take a long time to understand the origin of viruses.