单选题 Jazz and rock music Jazz and rock music are two main genres(种类) of the 20th-century music.They are popular with music lovers. Jazz evolved and became an important genre of music over the course of the 2Oth century,and during the second half of that century, rock music did the same. Jazz is an American musical artform that originated in the late19th and early 20th centuries in AfricanAmerican communites in the Southern Unit d States as a combination of African and European musical traditions. Jazz's West African origin is evident in its use of blue notes, improve is at in (即兴表演) , polyrhythms(复合节奏) ,and the swung note. From its early development until the present, jazz has also incorporated (吸收) elements from the American popular music. As it spread around the world, jazz drew on different national, regional, and local musical cultures, giving rise to many distinctive styles, ranging form New Orleans jazz from the 1910s, Gypsy jazz from the 1930s, bebop jazz(咆哮爵士乐) from the mid-1940s to jazz rap from the late 1980s. Rock music is a genre of popular music that developed in the 1960s from 1950s'rock and roll, rhythm and blues, and country music.The sound of rock often revolves around the electric guitar.Besides, a strong back beat(背景敲击节拍) is integral(必需的) to rock music, which is laid down by a rhythm section of bass guitars, drums, and keyboard instruments such as organ(风琴) , piano. Since the late 1960s, synthesizers are also employed into the rhythm section. In the late1960s and early1970s, rock music branched out into different subgenres, ranging from blues rock and jazz-rock fusion to heavy meal and punkrock, as well as the more cass ical genre of prog es ve rock and several types of experimental rock genres. Choose the best way to complete each of the following sentences. Rock music evolved and became an important genre of music during____