【正确答案】
C
【答案解析】[听力原文]
W: John, I"ve got to use your advice. You have been a builder for many years, haven"t you?
M: Well, that"s true. How can I help you?
W: The other day I was reading a book about roofs, and I got very confused. You see, I think I may have some repairs done on my roof, and I went to the library and borrow that book on roofs. I was trying to understand the basic principles of roof construction. You know I have been interested in this kind of stuff ever since my retirement.
M: What"s wrong with your roof?
W: The trouble is in what I think they call trusses, spelt T R U S S E S, and that"s one of the terms that confuses me.
M: Well, a truss is a structure with straight pieces forming triangles to support a load. When properly designed, these pieces are placed only under tension and compression and do not bend.
W: You mean a truss is a triangle structure that supports a load.
M: That"s right.
W: Also in the book I was reading, they talked about rises and spans.
M: Well, the distance between the centers of support is called the span. Rise is the distance between the highest point and base of the structure. The top members of the truss are called upper chords, while the base members are the lower chords. It"s really very simple.
W: Well, it may be to you as a builder. It"s really too complicated to me. Anyway, I have one more question for you. How much weight must the truss support?
M: First of all, of course, its own weight. Then the roof covering. In addition, depending on the locality, the truss must be able to support the weight of snow and the pressure of wind in severe weather conditions.
W: Aren"t trusses used in bridges, too?
M: Sure. Some suspension bridges have very long spans, as long as over 6,500 feet. By the way, the idea of the truss is a very old one. I read somewhere that, even in the Bronze Age, primitive men used trusses in their lake houses.
W: Well, I think I"ll be able to understand what I read a little better now. I shall not worry about being overcharged by you builders.
M: You must be kidding. You really shouldn"t.
Why does the woman go and ask John for advice?