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  3. Echolocating bats emit sounds in patterns — characteristic of each species — that contain both frequency-modulated (FM) and cons

Echolocating bats emit sounds in patterns — characteristic of each species — that contain both frequency-modulated (FM) and cons

admin2010-06-18  78
问题    Echolocating bats emit sounds in patterns — characteristic of each species — that contain both frequency-modulated (FM) and constant-frequency (CF) signals. The broadband FM signals and the narrowband CF signals travel out to a target, reflect from it, and return to the hunting hat. 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 pre, nee 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, their course in flight for the direction and velocity of their moving prey.
According to the passage, the shape of the target is reported to the echolocating bat by changes in the ______.
选项 A、echo spectrum of CF signals
B、echo spectrum of FM signals
C、direction and velocity of the FM echoes
D、frequencies of the FM or the CF signals
答案B
解析
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