33. 蝙蝠的回声定位

　　Echolocating bats emit sounds in patternscharacteristic of each speciesthat 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 echoesfor example, the targets size, shape, texture, surface structure, and direction in space. Because of their narrow bandwidth, CF signals portray only the targets presence and, in the case of some bat species, its motion relative to the bats. Responding to changes in the CF echos frequency, bats of some species correct in flight for the direction and velocity of their moving prey.

　　33.1. 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?

　　Only CF echoes alert the bat to moving targets.

　　Only CF echoes identify the range of widely spaced targets.

　　Only CF echoes report the targets presence to the bat.

　　In some species, CF echoes enable the bat to judge whether it is closing in on its target.

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