The starting point of herd immunity（群体免疫）for COVID-9 is still uncertain ，but many epidemiologists （流行病学家）believe it will be reached when between 60 percent and 80 percent of the population has been infected and develops resistance.A lower level of immunity in the population can slow the spread of a disease somewhat,but the herd immunity number represents the point where infections are generally less likely to turn into large outbreaks.
Studies that use the tests to examine only a small quantity of population,often called serology（血清学）surveys,are being undertaken around America and the world.The purpose of these new studies is to look for antibodies（抗体）in people's blood,proteins produced by the immune system that indicate a past infection.
An advantage of the tests is that they can pick out people who may have been infected and didn't know they were sick .A disadvantage is that the tests are sometimes wrong--and several studies,including a notable one in California,have been criticized for not taking the possibility of inaccurate results into consideration or for not representing the whole population.
While these studies are far from perfect,many countries,including parts of Sweden,are on track to achieve so-called herd immunity from the novel coronavirus-caused COVID-19 disease within weeks.In general,they give a better sense of how far the coronavirus has truly spread and its potential for spreading further.The starting point of herd immunity may differ from place to place,depending on factors like density and social interaction.



What do we know about herd immunity number from the text？ ______

A. The higher level of it,the better.
B. The lower level of it,the worse.
C. It is a point indicating a small number of infections.
D. It is a point indicating little chance of large outbreaks.

What kind of people can we get antibodies from？ ______

A. People who are sick.
B. People who are always healthy.
C. People who were once infected.
D. People who were killed by the disease.

What is the third paragraph mainly about？ ______

A. The contents of the studies.
B. Two sides of the studies.
C. The procedure of the studies.
D. The results of the studies.

Which of the following best describe the author's attitude towards the studies？ ______

A. He or she supports them.
B. He or she doubts their authority.
C. He or she thinks them perfect.
D. He or she partially believes in them.