大部分的语段都是由三个部分组成的：主旨句、扩展句和结尾句，主旨句用来点出语段的主题思想。找到语段主旨句后，就可与题目选项进行匹配，快速获取正确答案。下面我们通过两道托福听力真题解析，来具体看看如何利用语段主旨句帮助我们解题。

真题1
According to the professor, how is old computing different from new computing?
A. Old computing cannot be used to create complex programs.
B. Old computing tries to accommodate more types of users.
C. Old computing concentrates less on the needs of users.
D. Old computing focuses less on marking fast computers.
【听力原文】
Ok, so it’s really important to find out what people want, what’s useful to them, before we develop new technologies, which brings me to the next point about new computing: that designs should be not only useful, but also user-friendly. Besides usefulness, usability is important. The idea that computer programs, accessories, websites, really anything related to your computer should be easy to use. Sounds simple, I know. The term user-friendly has been around for years. The problem is we don’t always create things that are easy for users to use. Take websites: many of you have probably been on websites that were really confusing. You couldn’t find the information you were looking for or there was so much information on the site that you just gave up. Again, this is where thinking like da Vinci can help. The goal is to solve problems, not create new ones. Those websites might be really easy for their designers to navigate, but they aren’t the ones who need to use the sites. So again, designers need to involve the users from the beginning to find out their expectations and what kind of tasks they want to perform.
【答案解析】正确答案为C.
如上所述，该语段的主旨句为第一句话，即新的计算机设计注重客户需求，正好对应C选项：Old computing concentrates less on the needs of users. 传统的计算机设计不太注重客户需求。

真题2
According to the professor, what determines the temperature of the barriers in the facility’s basements?
A. The amount of heat generated by the facility’s solar panels
B. The amount of heat transferred from the facility’s windows
C. The temperature of air drawn in from other locations
D. The temperature of the surrounding soil
【听力原文】
And it only gets better! You see under each of the buildings, architects designed a broad, shallow basement space that’s kind of like a maze. Air, either from within the building or from the outside, is pulled into the basement space through air ducts and then forced to move slowly around concrete barriers. The air is sort of guided through the maze. Now these concrete barriers, they absorb heat from the air if the air is warm or if the air is cold, the cold is absorbed into the concrete, so it could be either way. Let’s say it’s hot air; hot waste air being pulled in from the computer rooms, say. Well, that air moves slowly back and forth around these barriers. In this particular situation then, the heat from the incoming air is transferred to the concrete, which then holds onto it, storing the heat until later when it’s slowly released throughout the building. That’s the concept of thermal mass in a nutshell.
【答案解析】正确答案为C.
该语段的主旨句是最后两句。大意是空气中的热量被混凝土所吸收并储存起来，随后慢慢地将热量释放到建筑物内。正好对应C选项：The temperature of air drawn in from other locations 。