GRE阅读练习每日一篇(三十九)_GRE阅读

GRE阅读练习每日一篇(三十九)

　　Symphonic music derived from complex jazz motifs

　　An experimental novel based on well-known cartoon characters

　　A dramatic production in which actors invent scenes and improvise lines

　　A ballet whose disciplined choreography is based on folk-dance steps

　　A painting whose abstract shapes evoke familiar objects in a natural landscape

　　Echolocating bats emit sounds in patterns characteristic of each species that contain both frequency-modulated and constant-frequency signals. The broadband FM signals and the narrowband CF signals travel out to a target, reflect from it, and return to the hunting bat. In this process of transmission and reflection, the sounds are changed, and the changes in the echoes enable the bat to perceive features of the target.

　　The FM signals report information about target characteristics that modify the timing and the fine frequency structure, or spectrum, of echoes for example, the target s size, shape, texture, surface structure, and direction in space. Because of their narrow bandwidth, CF signals portray only the target s presence and, in the case of some bat species, its motion relative to the bat s. Responding to changes in the CF echo s frequency, bats of some species correct in flight for the direction and velocity of their moving prey.

　　24. According to the passage, the information provided to the bat by CF echoes differs from that provided by FM echoes in which of the following ways?