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.

Which of the following is TRUE of CF echoes？

A．They enable the bat to determine the size of the target.

B．They help identify the range of widely spaced targets.

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

D．In some species, they help the bat to tell the shape of the target.