Since December, there have been tens of thousands of COVID-19 cases both in and out of China, caused by a previously unknown virus: novel coronavirus.
    It’s not the first time for viruses to make headlines. Viruses have brought big trouble to humans, with their potential to cause widespread disease and death.
    What are they? Viruses are considered to be non-living organisms (有机物). They can infect animals and plants, making them sick. They contain genetic materials like DNA and are protected by a coating of protein.
    Unlike bacteria, viruses can’t reproduce (繁殖) on their own. Instead, they invaded the cells of living organisms to reproduce, spread and take over. 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 virologist at the university of Cape Town in South Africa, told Scientific American, “because viruses don’t leave fossils (化石) and because of the tricks they use to make copies of themselves within the cells they’ve invaded.”
     Understanding the origins of viruses means fully understanding the history of their hosts – not only humans, but also bacteria, plants and other animals, which is a complicated task, says Nature. However, there are three main theories (推测) to explain the origin of viruses. First, viruses started as independent organisms, then became parasites (寄生者). Second, viruses evolved from pieces of DNA (脱氧核糖核酸) or RNA (核糖核酸) that “escaped” from larger organisms. The third one is that viruses co-evolved with their host cells, which means they existed alongside these cells.
   7.8  For the time being, these are only theories. The technology and proofs we have today cannot be used to test these theories and discover the most plausible 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.
5. What can we learn about viruses from the article?
A. They mainly infect animals and plants.
B. They are really small living organisms.
C. They are more dangerous than bacteria.
D. They can’t reproduce unless they find a host cell.
6. Which of the following might explain the origin of viruses?
A. They evolved along with their host cells.
B. They evolved from the DNA or RNA in animals.
C. They evolved from the fossils of large organisms.
D. They evolved from parasites to independent organisms.
7. The underlined word “plausible” in the last paragraph probably means “_______”.
A. creative  B. reasonable     C. common  D. unbelievable
8. What do we conclude from the article?
A. Viruses live longer in human host cells than in animals’.  
B. Viruses will become more like bacteria as they evolve.
C. It may take a long time to understand the origin of viruses.     
 D. The author is confident about future virus research.

5---8  DABC