A、Denim jeans. B、Informal jeans. C、Jeans with zippers. D、Blue jeans. D录音开头提到，牛仔裤有多种款式和颜色，其中blue jeans(蓝色牛仔裤)被认为最具美国特色(特别是美国老西部)。

Jeans are pants, or trousers, made from denim. Mainly designed for work, they became popular among teenagers starting in the 1950s. Historic brands include Levi’s and Wrangler.
    Jeans are now a very popular form of casual dress around the world, and have been so for decades. They come in many styles and colors: however, "blue jeans" are particularly identified with American culture, especially the American Old West. The American population spent more than $14 billion on jeans in 2004.
    Initially, blue jeans were simply sturdy trousers worn by workers, especially in the factories during World War II. During this period, men’s jeans had the zipper down the front, whereas women’s jeans had the zipper down the right side. By the 1960s, both men’s and women’s jeans had the zipper down the front.
    After James Dean popularized them in the movie, Rebel Without a Cause, wearing blue jeans by teenagers and young adults became a symbol of youth rebellion during the 1950s. Because of this, they were sometimes banned in theaters, restaurants and schools.
    During the 1960s the wearing of blue jeans became more acceptable. By the 1970s they had become a general fashion in the United States, at least for informal wear.
    Notably, in the mid-1970s the denim and textiles industry was revolutionized by the introduction of the stone-washing technique by GWG(Great Western Garment Co.). This helped to bring denim to a larger and more versatile market. Denim suddenly became an attractive product for all age groups. Acceptance of jeans continued through the 1980s and 1990s to the point where jeans are now a wardrobe staple, with the average North American owning seven pairs.
    Jeans can be worn very loose in a manner that completely conceals the shape of the wearer’s lower body, or they can be snugly fitting and show the body. Historic photographs indicate that in the decades before they became a fashion trend, jeans generally fit quite loosely, much like a pair of bib overalls without the bib. Indeed, until 1960, Levi Strauss denominated its flagship product "waist overalls" rather than "jeans".
20 What jeans are particularly identified with American culture?
21 Why were jeans banned sometimes in theaters and restaurants in the 1950s?
22 What was the initial style of jeans before they became a fashion trend?