incorporated(并入) into an HIV drug with broader applications. It's even possible that it could someday be adapted to reduce the risk of HIV transmission in adults as well as infants.

语篇解读　本文是一篇说明文，介绍了一些科学家对母婴传播HIV的研究活动及相关进展，向人们展现了人类战胜疾病的美好前景。

4．What is the main idea of Paragraph 1?

A．The breast milk from an infected mother doesn't contain HIV at all.

B．Only a small percentage of infants breastfed by infected mothers catch HIV.

C．The possibilities of infants catching HIV have nothing to do with infected mothers.

D．Children feeding on infected mothers' milk won't catch HIV though exposed to it often.

答案　B

解析　段落大意题。根据第一段第一句中的"only about 10－20 percent of infants who are breastfed by infected mothers catch the virus"可知，虽然是感染了艾滋病病毒的母亲为婴儿哺乳，但这些婴儿染上艾滋病病毒的概率很小。

5．Why did the researchers divide breast milk into smaller fractions?

A．To find out Tenascin C.

B．To make up specific proteins.

C．To add them to a tissue culture.

D．To test what prevents cells from being infected.

答案　D

解析　细节理解题。根据第四段第一句中的"and tested which of these fractions，when added to a tissue culture，prevented the cells from being infected by HIV"可知。

6．Which of the following about Tenascin C is TRUE?

A．It has become an effective tool in fighting HIV.

B．It is the key protein on HIV's envelope.

C．It can prevent transmission in a live animal effectively.

D．It helps prevent mother­to­child transmission of HIV.

答案　D

解析　细节理解题。根据第二段第二句"Human breast milk naturally contains a protein called Tenascin C that neutralizes HIV and，in most cases，prevents it from being passed from mother to child."以及第五段内容可知，Tenascin C可帮助防止母婴传播HIV。