听力原文： OK, so in our last class we were discussing big bands swing music. You remember this was a kind of dance music with a steady rhythm. (26) But today we deal with music played by smaller jazz bands. It's called bebop and (27)it may use all sorts of new types of rhythms, some of which are very irregular. We'll talk more about that later. (26)But first I want to talk about some of the social elements that I believe contributed to the development of bebop music. To do this, we have to look at when bebop arose and started becoming so popular, which was from the late 1930s through the 1940s, from the time of the environment for bebop music was the decline of the US economy. During the great depression, the economy suffered tremendously. And fewer people had money to spend on entertainment. Then during World War II the government imposed a new tax on public entertainment, what you might call performance tax. (28)The government collected money on performances that included any types of acting, dancing or singing, but not instrumental music. So to avoid this new tax, some jazz bands stopped using singers altogether. (29)They started relying on the creativity of the instrumentalist to attract audiences. (28)This was what bebop bands did. Now remember a lot of bands have singers. So the instrumentalists simply played in the background and had occasional solos while the singer sang the melody to songs, but not bebop bands. (27)So the instrumentalists had much more freedom to be creative. So they experimented, playing the music faster and using new irregular sorts of rhythms. (33)